Would it still work if i bought a pre owned version and used a different code instead of the orginal?

i bought the Pre-owned version and i am going to use a different code to activate my account. so far i have asked the shop keeper from Blockbuster and hes said it would work. next i asked a shopkeeper in a little game shop called Game street, he also i agree with the fact that its possible. therefore people that have done you can get your hopes slightly up, considering that the cost of a code is just �9 or $9. Finally i just wanted a little more evidence before i waste my money. what do you guys think?