Now then-- EF.... man.... you know you have kind of a short attention span and you know that if you sell the game back, you're going to regret it. Beyond the fact that you're probably not going to get more than $5 for it anyway, there's just no reason to do it. Sooner or later, you're going to want to play it again, and then you're going to be on here wondering where you can get another copy and lamenting the fact that you sold your last one. Keep the game, set it aside, play something else for a while. Maybe you'll get another hard drive or figure out a way to hook this one up to the 'net long enough to get working GotY content-- maybe you'll just get the urge to play the vanilla content again anyway. In any case, for what you'll get seling it back, it's just not worth it.
It took me five tries, over the course of more than a year, before I really got into this game. Just don't worry about it.