ok so i have an iphone now and im like so sick of it and whateverr. but i really wanna sell it on ebay (its the old generation, so it can be unlocked) for about 340$$. I got it on October 13th 2007 and I am under contract for two years I think. What happens to my contract when I sell it on ebay? Will I have to void my contract with at&t?

thanksss<333