I have a question. Do film agencies or clients check licenses for Prores codecs?

I have some videos in AVC/MP4 format, if I convert the videos to ProRes codecs through non-certified programs like Shutter Encoder or other programs that use the ffmpeg library, is it legal?

Who is responsible for selling movies in ProRes codecs. Film agencies, author or client?

I also have Vegas Pro with certified ProRes codecs but AVC/MP4 movies exported to ProRes are not of good quality with artifacts and not acceptable.

Better image quality is in free or paid converters that do not have a license for ProRes codecs, but what about the license, as Apple writes on its website?

if your original file is using a 10bits codec, then you can do your customers a favor by converting and uploading ProRes, since (unfortunately) it's the only universally accepted codec able to preserve 10bits (I use Adobe Encoder, which is part of the free package offered by AS).
If your original file is 8 bits, it doesn't matter.

But I don't think that the vast majority of buyers will care too much about the codec. They will look for the right content first.

I have a question. Do film agencies or clients check licenses for Prores codecs?

No

I have some videos in AVC/MP4 format, if I convert the videos to ProRes codecs through non-certified programs like Shutter Encoder or other programs that use the ffmpeg library, is it legal?

Legal. Everything that is not prohibited is legal.

My main work is as a senior (never liked the word!) editor in Broadcast TV.
If someone searches for ProRes, it is because he has his reasons.
No, they are not naive to think it's better quality.
They know how it goes.
Why are they searching for ProRes?
Because no matter the original file (8-bit 4:2:0 or 10-bit 4:2:2) the video is locked in a codec with no visible copy loss.
Apple says that ProRes HQ has that benefit, and then plain ProRes is ok.
Many never think that all videos are compressed (there are better compression codecs though) and since videos are compressed, every time you process them you get a slight loss of quality.
Here is how it could be:
We record (in any format) and that's the original file.
Then, we process and export for micro, which is the second copy.
After that,  the buyer gets the file and edits it, which is the third copy.
In the end, he exports the finished product and that's the fourth copy.
There might be a fifth copy if someone who gets it wants to add subtitles, or export to another codec.

Therefore, a high-end customer will prefer ProRes if he has the option to choose from the same content.

Personally, I upload all my videos to ProRes because I have some high sales to the big three and I once (I might have said that in another post)
had a ProRes sale that earned me $603, which means the customer paid over 3 times more.

Final note: if I export an 8-bit to 10-bit (since I process my clips in post) even if that's just H.265 4:2:0 10-bit or ProRes that always locks to 4:2:2 10-bit I never see banding, which sometimes was the case with H.264.

But I will have to agree that these high sales are not frequent, and each one has to decide separately.
Exporting in ProRes means more time to upload and way more needed space to backup.

I read that the "mov" format is better than "mp4" in quality. If the buyer is not looking for "prores", then when choosing between "mov" and "mp4" in the H.264 codec, most buyers will choose "mov".