I can't explain the technicalities of how it actually does those thing (I'm guessing it wouldn't be that interesting/ understandable anyhow) but I can vouch for the WooCommerce set of plug-ins.  They are very well supported and are generally top notch.  The media file download feature isn't new so I'm guessing the have the bugs pretty well worked out of it.

I just researched woocommerce yesterday for digital downloads for microstock. I have used woocommerce for selling products, and it is a very good plugin for that. But it isn't set up for multi-sized digital downloads, that I could see, (yet). There isn't any way, that I could see, to serve up to the customer the size they order. In other words, woocommerce doesn't have a separate protected folder where the original sized images are uploaded to, then when the customer orders a different size, it doesnt auto size the image and serve up that size to the customer, like the Sell Media plugin does.

It IS, on the other hand, set up for photographers selling digital downloads of a certain size. And you can impose a download limit or time expiration on allowing downloads.

Here is a link to the documentation:
http://docs.woothemes.com/document/digitaldownloadable-product-handling/

I was hopeful when I started reading, and even installed the plugin and played around with it, but no dice.

Here is a link to more documentation. If you scroll down under the tshirt photos, it even says this:
"Please note: This extension does not help with selling digital photo downloads." I took that to mean no stock images with multiple sizes.

http://www.woothemes.com/products/woocommerce-photography/

If I am wrong about any of that, please say so. I would love to use it for microstock out of the box.

And no, I will NOT get involved with the whole symbios/grfx/leo/robin mess. Been there. Fool me once, shame on you. Fool me twice, shame on me.