Specifying what you’re looking for may help those who have info for you.
1) Which carrier? (ATT, Verizon, etc)
2) Locked or unlocked?
3) Version preferred? Minimum version required? (4, 4s, 5)
4) Memory size preferred/required? (16, 32, 64)
5) Condition preferred/required? (like new, decent, etc.)
6) Warranty length preferred/required? (n/a for a private buy)
7) Would you be willing to buy one with a plan?