how do you know if a used cell phone from sprint can be activated on your account?
i want a new phone but don't want to extend my contract. i also don't want to pay 0 or more for a phone. can i buy one from ebay and have it activated to my sprint plan

Any phone that has the "Sprint" logo is qualified for sprint.
If it doesnt have a logo it can not be used on the sprint network.
And you can buy them anywhere that sells sprint products.
-ebay
-Srint
-Radio Shack (Sells used and Recon phones too)
-Best Buy
-Srint Express (Sells used phones too)
-Wal Mart
Here is a list of phones you can use that are Sprint Approved.
http://www.phonescoop.com/carriers/carrier.php?c=1
http://support.sprint.com/devices.html?isd=1&&navtypeid=2&pagetypeid=35&prevPageIndex=1