Swappa has a fairly decent community and staff, and they only charge a flat $10 fee for their services (excluding PayPal fees of course).
I've had nothing but positive experiences on Craigslist as well. Try to negotiate the final price or trade before meeting up, as some people will try to stiff you and ultimately waste your time. If you're selling the phone, keep it on your terms (you designate the location and time) and ALWAYS meet up in a well-lit, public area. I personally like to meet at Starbucks, as it has seating for the buyer to inspect and test the device.
Oh, and bring a SIM ejection tool. I made the mistake of selling my iPhone 4 to a person who scratched up the bezel by trying to use a staple to remove the SIM tray. Ugh.