buying a used pc game on ebay, serial number works when online?
 

I'd like to buy a copy of AOE 3 but I don't want to pay the full retail price. What happens when you buy a used copy of this, will it work online or does every serial number get registered to one IP or something like that? I just want to make sure I don't run into problems after buying a used copy.

You could play as long as the person that used to own the game isn't online at the same time as you are. Also, if it's like steam, you might have to get their account information because when you create an online account with steam, you have to have a valid, unused cdkey which binds to that username.. That's how steam is, I bet aoe isn't like that but I am not sure.

make sure you get the UPC code for the game if possible, if not, just take a digital picture of YOU holding the CD with the key clearly in view, then send that pic as an attachment to whichever game maker's customer or technical service department. Essentially, you will be registering the game under your name, and if you have any problems with other people using that CD key, they will usually remedy it ( e.g. banning the current CD key and issuing you a new one )
I had to do that when I bought a used copy of Myth: The Fallen Lords, and Everquest:Ruins of Kunark.