There’s a few different phone sales sites out there, but Swappa is focused on Android. You can sell your new or used phone, or purchase from other listings. It’s less messy than Ebay or Craigslist. Being a brand new site, there’s only about 50 listings right now, but it’s growing.
I actually just listed my (brand new) MyTouch Slide on there. The signup process is super easy letting you log in with Facebook or OpenID, I used my Google Account. Pick a phone category and add a description, condition of the phone, and other details. There’s even an option on there to note if the phone is rooted and has CyanogenMod installed. This is definitely a site by Android lovers for Android lovers. And the fee is reasonable, only a flat $10 is paid to Swappa when the phone is sold, payment to the seller is made via PayPal. Compare that to the $40-50 you might pay to Ebay for a top dollar phone, or the low ball prices you may get from Craigslist.
Pricing is about on par from other used phone sales venues, but still you may find a really good deal, or be able to sell your phone quickly.