Yeah, the system would be built so that it'd be easy to keep an inventory and add more codes/cdkeys as you get them, for one-time sales like a copy of a game, or to set a product as unlimited if it's a digital download that you can resale like an ebook or something.  Glad to hear it might be of interest, I'll PM you if and when I get the basics of the site up and running so you can check it out.

Also please PM me if you get a hold of any civ 5 copies and how much it'd be

Out of curiosity, do you have a list of games you currently have in stock and for sale?