I think the limiting factor is not going to be the cart software but the amout of disk space you have available for your database. Shopping carts use the database to store product information. As far as the cart software itself is concernced your products are nothing more than a db record number.
It seems like eBay and Amazon have the platform to host such a beast. You have to ask yourself how much content will you have each record and will the pages really be unique enough to not trigger duplicate results filter. I'm thinking this data you are using is already published on the web in some form?
As rightly pointed out the hard disk space could be a limiting factor. Mysql 5 is a beast and can handle a large load. Also are you looking for opensource shopping cart? If so you may want to look at www.oscommerce.com