But I really think this idea is at best a pipe dream and at worst a licensing disaster.
To highlight this lets look at Flickr. That lets you choose a free (Creative Commons) license for your material. So Commons regularly grabs useful images from there to host. And; we still get people complaining that they didn't understand this meant you could take their image and upload it somewhere else.
And this is from a site where the whole point is to upload images.
Now lets imagine Ebay implemented this; the idea that most of that sites users would comprehend the extent of a little tick box is, I feel, minimal.
I obviously am 100% behind freely licensing content - and do a lot of the sort of work he mentions in getting image/content releases by email. But I also dislike the idea of essentially misleading people and then telling them they have no way to go back on the license (yes, this happens).
I might be an outlier here in my criticism. But Commons, in my experience, tends to treat non-Commons people (especially image copyright owners) like crap. That is what needs to be fixed first, before any pressure is applied to companies like Ebay...
I feel this is unfair. They treat everybody like crap.
On the other hand, they are very scrupulous about obeying licenses.
Anyway, the underlying assumption of the OP is that Wikimedia Commons actually wants more images. Surprisingly, this is not the case, at least, it's not a universal sentiment among Commons volunteers. Many of them believe they are curating a set of the best free images, not making an indiscriminate collection. Some see Commons as just a set of images to illustrate Wikipedia with, and don't believe it has a separate mission to collect other free imagery.
Also, Commons is already overwhelmed policing the content that gets uploaded. Unlike Flickr or other image hosts, there's no mission of self-expression there; instead, they believe every image must be correctly licensed, well categorized, and useful to someone else (and not spam or otherwise inappropriate). What's needed right now are more community tools for managing the influx.
You're right that there's no way we can explain CC licensing to eBayers. We can't even get that right on Flickr, and there are significant problems even on Wikimedia Commons itself. No matter what you tell people, make them click on, whatever, they persist in a primitive folk belief - by creating the image, they get to control who does what with it in perpetuity.
One of the big problem with many of the eBay auctions is that they don't use original material - for example, if you're selling a copy of a game or movie, often you won't bother to take a picture of your copy - you'll search online and find the best one you can, slap it in the auction and call it a day.
This happens all the time, and as auctions are very time limited, it often doesn't raise the ire of the original source of the media.
If they had some sort of dedupe/anti-"stock photo"/anti-"professional product shot" thing in there, this could probably work.