You have alot of choices out there however just keep in mind a few things that could save you and the seller from misunderstandings.  Always go by eBay's standards, LOOK at the feedback.

 Make sure that the seller has a return /refund policy in the event that you do get a phone that is defective,  you will have that much more assurance that you will not lose anything.  If you do have a problem give your seller time to correct the problem and contact them before leaving any feedback. If you still have a problem,  file a complaint with paypal, use negative feeback only after you have done these things or you may find yourself stuck with a retalitory negative . 

Once you find a reliable seller make sure the phone is compatible with your wireless carrier.  If your phone uses a sim card,  it is a GSM or iDen phone  (iDen is Nextel)  and it  will not work with a carrier like Alltel or Verizon.  If you have Verizon or Alltel make sure the phone you buy is GPS or e911 enhanced, manufacturers of cell phones are now required to produce only e911 compatible units.  Alltel and Verizon will now only activate those with e911 capabilities.

Many of the oldie but goodie phones such as the Motorola Startac are no longer activated by Verizon or Alltel because they are not e911 enhanced., however if you still have the much beloved and popular Star-Tac or any other older unit,  do not panic! The law applies only to newly manufactured phones.

Once you find the phone you like, CALL your carrier and confirm that the unit will work with your service.  Ask the seller for the"esn" or electronic serial number and be prepared to give it along with the make and model of the new phone to your carrier.