Let's start with what we already have:
We use X-cart and we also list on eBay. The problem we face, is that customers do not like to be told to "go here and checkout". First, it makes them think of the possibility of being ripped off or something. Second, they have to input information all over again. THEY HATE THIS.
Now, there are several products already on the market similar, but nothing with all functionality we are looking for. One program is Auction Manager for X-cart. It has most of the functionality we need, but not the most important part, and that is the checkout function.
So, what we need to do is have a mod built for Xcart. This mod would help us to create the listings for ebay, and maintain them from within the xcart admin. Add to that, the ability for xcart to capture all important information regarding the customer, automatically plant it within xcart, complete the checkout, and return the customer back to ebay again. This should actually be a very easy job for an experienced programmer.
The module should be built using templates already contained within xcart. I would imagine that you would have to contact ebay via whatever method it is that they use so that you can obtain API keys or code.
I'm still working the details out in my mind, so please contact me with any questions you may have through scriptlance.