Direct From the Steam Wiki
Games bought through Steam cannot legally be resold due to the unsuitability of current proof of purchase laws. The only valid proof in an entirely digital transaction is the credit card used during the process, but as ownership of credit cards cannot be transferred, neither can that of Steam-bought games.
When a buyer purchases a boxed game they must authenticate it with the registration of a CD Key. In the event that the CD Key that they have registered is already in Steam's database the user is required to submit an image of the physical purchased CD Key for verification purposes, as well as a purchase receipt. Valve does not accept receipts from eBay or known used-game resellers
I suggest you read the entire wiki section, as they are also doing many other shady tactics wither their users..
Valve/Steam based games should be avoided at all costs.. Bottom line is you got scammed, or the orig. user/seller didn't know
about this restriction.