Depends on what site you use. One's like Marquee Dragon are basically just a way to find people that want to buy your account and the rest is up to you. Paypal/AIm and get it over with. Unfortunately, it's really really easy to scam people both ways. You can just call up blizzard the next day and say your account got stolen, or you can just say someone illegitmatly transfered money off your paypal account.
You just have to be careful on that site. Teh minute you put up your account as a thread, you'll get bombed with people trying to scam you, and will offer you retarded armounts of money for your account. Those are the people you don't want to deal with. Honestly, your best / safest / quickest bet is to just sell it to a site that buys accounts all the time and sells them back. You'll get a lot less, but you'll have a lot less hassle + no chance of getting it scammed out from under you.