i heard a shiur about it once. The rav wasn't really familiar with eBay ( he was using it as a verb " when we ebay we are...) quite annoying. Anyway it should be מותר because you aren't doing anything but obviously ask your own שאלה.
There are תשובות from years ago abut owning a vending machine. That is a lot worse because your כלי is being קונה the money on Shabbos and still יש להקל .
With a vending machine the terminology used, in at least one teshuva that I saw, was that the machine is in a reshus harabim and the inventory is not in your reshus etc. unlike ebay where the inventory is probably in your house.
I am totally clueless in this inyan but I would be cautious before being medameh milsa lemilsa.
Always ask a competent LOR!