This is something that I would love to see OSC have in the near future. I am not good with PHP, but am learning. It would seem to me that the best place to start would be figuring out how to get data from eBay. This could be done by a PHP script and then the relevant data inserted into the database creating a product with a stock level of one, so that when the user purchase the item it is changed to a stock level of zero.
I did a little searching and found a script that rips data from ebay and creates variable from the data. The script doesn't currently work, but it should give some ideas as how to get data. Once you have the data, inserting it into the database is easy. You would need a way to run the script when the auction closes, I would assume via cron. I know that eBay does allow for other other online payment methods when you list an item although I have never tried this to see what the options are when listing an item.
Here is the script that I mentioned: http://www.aeoninteractive.net/scripts.php
Just thinking out loud