yikes, don't buy PC games used!
If you want to play it online, the other person has already made a username/password for it and you'd have to use theirs. Also there would be some concern over the Product Key being used more than once, but it might not be a problem.
For games which have a monthly subscription price, they never ever seem to drop.
I don't know how soon prices for regular titles drop, but look around in different places, some will overcharge.
And then there are digital downloads, which are games that you pay for online and you download the game and are given a product key. Since you don't get a DVD, paper manual, packaging, shipping, it's cheaper to get games this way.
Steam sometimes will have good sales on games if you watch and want to get a game attached to a Steam account.