Issues with my iphone 4s and looking to by an unlocked iphone 4s

I recently have had issues with my iphone 4s and have buy a new one. I am currently on an AT&T plan and am not eligible for an upgrade yet and I'm wondering if any unlocked iphone 4s will work with my currently AT&T plan if I have the right SIM card. I don't want to pay full price for a brand new unlocked iphone so I'm looking to get a used one. I'm also wondering if there any hidden fees i'll encounter by doing this like activation fees when I get a new phone and try to activate it on my current plan. If anyone knows of legitimate websites to buy fair-priced used iphones I would appreciate the help with that as well. Thanks.