Recent ebay final sold prices seem to be running 350-500 used 16gb, 500-650 nib 32gb unlocked. 500 seems about average. I don't think you're going to see 300 anwhere, esp for nib, but you never know.
Craigslist. Be sure to call vzw and give them the imei # first to see if phone is unusable. Some will sell phone after ducking out on their contract and vzw will "lock" the phone rendering it absolutely useless forevermore.
At least with craigslist you are able to see what you are buying before you potentially buy a bricked or locked phone.