Use craigslist, I usually take the used price people would pay at gamestop and take like $5 or $10 off that. For example I bought Assassin's Creed Brotherhood on sale for $45, and then sold it on craigslist.com for $45. I played the game brand new for free.
I looked at Gamestop and their used price is $43. So $35 or $40 is a good price if you're selling it.
Well if gamestop said $30, thats good. I took Homefront back to the same Gamestop I bought it from 6 days after release and they only offer $20 for trade in! Now THATS a ripoff! The games wasn't even a week old! I then sold the game on eBay for $41. I would put the game on eBay and set the price for $39.99, if it sells at all, at least you would get most of your money back.