In Electronics Boutique and GAME stores (in the UK, at least), you can determine how much they rip both the buyers and sellers of pre-owned games off quite easily.

On the back, over the UPC symbol, they'll have written a four digit number. The last two digits are what they paid for the game.

So, if the code is:
then whoever sold EB the game got £15. And you're about to pay £19.99 for it. Well, all those brain-dead, games illiterate employees have to get their wages somehow.