There are graphic stores that help you create motion graphics with resources of textures, models, special effects generators under royalty free or CC0 licenses.

Is it legal to create stock video works from elements of other authors? Do you have to be the author of all elements in motion graphics?

Depends on the agency. However, since I am probably one of the few (based on experience and questions I've seen other people ask, etc) that actually reads license agreements - most agencies yes, do require you to be the actual 'creator' of the assets.

In certain circumstances, which I think your question is - (i.e., let's say you create a 'scene' composed of 3rd party assets of couches, chairs, light bulbs, etc) - as long as the derivative work is significantly different AND you have the 'resell' license (i.e., you can sell derivative works without attribution/etc) - then usually you are okay. It is only if say you get a "license" of a chair - and don't modify it at all and just put it on a black background that that is a big no-no (i.e., trying to sell/pass off someone else's asset as your own).

Which specific agencies are you concerned about? You should read their license agreement and/or reach out to them for clarification of what types of assets they are okay with selling on their platform.

Thank you for your quick response. Yes. It would be better to ask the authors of add-ons. Just what are texture add-ons, planets, cloud generators, nebulae, plants in blender that are on sale under Royalty free license for commercial use for? Only for games?

Ready-made 3d models don't interest me. I know that you have to be the author of the 3d models. I'm more interested in textures and effect generator add-ons that make it easier to work to moving graphics. Like for example creating landscapes. In stock agencies, for example, there is a lot of our planet without NASA's signature. Where do authors of moving graphics get such textures?

Free Blender or Unreal Engine also have add-on stores. It is difficult to create moving graphics without add-ons. I just don't know which add-ons for a fee you are allowed to use for your moving graphics to sell.

Is a derived resell license an extended license?

Rather, the popular stocks such as Istock, adobestock, shutterstock, pond5, envato, etc.

a) A 'derivative' work simply means that you are making something 'new', instead of selling the asset 'as-is'. For example, if you got a 3d model (commercial use license) of a toaster - that you would use the toaster saying in a 'kitchen' scene, complete with fridge, fruit basket, table, chairs, etc, as opposed to just using it as a "toaster".
Most assets (in their license) do not allow you simply 'reselling' them 'as-is' - but require you to be used as 'part' of a bigger scene. As long as the asset itself is not the "main focus" (i.e., the only thing people focus on) when looking at it, generally speaking it is okay.

b) For texture map generators, cloud generators, etc - generally speaking that is okay as well - simply because the maps/animations/etc that are created are unique and require a 'seed' to create it. For 'nasa' pictures (which, btw - many of those are actually computer generated) - I believe (if I recall correctly) NASA has actually said a number of images are for 'public' use, etc and so people tend to just use those as texture maps on a 'curved' surface, etc - thus creating a brand new unique item. But of course - you want to make sure the image is public/commercial use/no attribution required/etc.

c) Add-ons (from experience) - provided they are used in the 'creation' of the asset - are generally speaking okay.

You want to make sure the license (when you purchase it) says that you have a 'royalty-free commercial use' license for it.

The agencies you mentioned - 'generally speaking' - should probably be okay for the types of use you have described. (I.e., you are not selling the 'add-ons' as standalone products, but rather using them as part of the creation of brand new 'derivative works' assets). As long as it is not obvious it is the original 'asset', etc and you have the proper license, generally speaking it should be okay.

If however you have a very specific question, I would recommend reaching out to whichever agency you are concerned about and asking them. Most likely they would say the exact same thing I just said above, but - they might have some further insight for you as to which asset types/etc they do/don't accept.

But bottomline - like I said - make sure you have a 'royalty-free commercial use' (no attribution, i.e., saying 'xyz person made this asset') license for any add-ons/etc that you use, and you should probably be okay.

It's nice that there are still helpful people on the forum. Thank you very much.

I still have a question about point a)

If we buy for example a package of realistic clouds in VDB format under Royalty Free license. And we make it a moving graphic with many clouds by moving the camera or clouds, for example, is that okay? Does there have to be another object besides the clouds to be able to sell moving graphics?

Another example. If we buy the hdri texture alone under Royalty Free license for a spherical planet and make it a moving planet in moving graphics without other objects, is that okay? Or must there be additionally another object?

It is also possible to create interesting textures for moving graphics thanks to AI but I prefer not to use this technology, not all stock agencies accept partial input of AI in moving graphics as in AdobeStock. I prefer to buy authoring elements for graphics and support the author.

I know that he should write to the agency. For now, I wanted to get some advice on this interesting forum.

Generally speaking - one uses common sense. Think if YOU created the asset - and someone else created something with it. Whatever 'they' created - would you be okay with them reselling it? If not - then you probably shouldn't either. If yes, then it would probably be okay.

In your example - if all you did is have 'two' clouds that didn't intersect (i.e., say two separate clouds on a black background) - it would probably "not" be okay - because the original asset was in its original form. However, let's say you made multiple clouds, different shapes/sizes, overlapping, etc - then it "might" be okay. If it still looked like the 'original' asset (i.e., someone could figure out easily what you used to make it) - then probably not okay. But if it looked brand new/unique, unidentifiable - then probably okay.

For textures - generally speaking you have a little more leeway because you are creating a brand new asset, in which the original asset could not easily be extracted (or identified) by someone else. I.e., if you took a flat HDRI texture, and mapped it to a sphere (i.e., say an earth), then further added say some clouds, or light effects, etc that affected the shape/size/color/etc of the texture mapped onto a sphere, then you would probably be okay.

Basically, ask yourself a few questions:
a) Is it easy to identify the original (purchased/licensed) assets in your works? If it is - then it is probably "not" okay to use as-is.
b) Could someone easily 'extract' the original asset (purchased/licensed)? If they can - then it is probably "not" okay to use as-is.

When creating "derivative works" - you want to create something "brand new" that does not at all resemble the original assets. If you do that - then you are probably okay. And yes, ask the agency for further clarification if you like - but they will probably tell you the same thing.

Yes I know that it is better to mix multiple clouds. Just what if there is one cloud and we render with camera for example rotation and some other effects with camera. It's still not a VDB file sale like a graphics motion render. I guess it is better not to make renders of one cloud as to mix many as you wrote and I agree.

As for hdri textures there are ready-made planets for sale with clouds too. I don't know if after buying such a texture you can do graphics renders of the planet itself with camera effects or there must be an additional other graphics element as well.

Out of curiosity I will also ask the authors of these graphics.

In blendermarket graphic elements do not have an extended license in addition to the usual Royalty Free, gpl or mit? Isn't such an extended license needed for a stock motion graphics producer? After all, motion graphics also sell many times. 

Also, how is it with those free Blender tutorials on Youtube? There are a lot of tutorials like abstract backgrounds or shapes and they are almost very similar to those on stock sites. Someone learned thanks to the tutorial or they download free shaders, nodes, blend files and then they render it further to stock agencies. Is this in order?

Like I said - easiest rule of thumb is probably this...

If it looks like you copied it from somewhere (i.e., youtube/3d model site/etc), don't use it/don't submit it - because you probably haven't made an 'original' work, probably someone somewhere will get annoyed, and it could potentially cause you some issues down the road.

If it looks original (and it is of course original), and you can't tell where the original assets came from (and of course, you have appropriate licensing rights/etc), then it is probably fine...