Hi. i have a 1600+ AMD athlon xp and i wanted to upgrade if i found on on ebay a xp 2500+ and it says sold as is dont knw if it functional or not..well im taking the risk in buying but If it was a non working chip and i installed it would it blow my comp up or do somthing or could i just install my old chip back?

It all depends on your motherboard. What make and model do you have?
Use the "What's inside my PC" link in my signature if you don't know.

well i knw what my motherboad needs im just using the athlon xp ones..which work i think its 462 pins but..i wanna knw if i buy a CPU off of Ebay and the seller says its unkown its it works or not.. i wanna knw what happens if i install the used chip and it dosent work? will it be harmfull if i just put my old on backk?

Sorry for the delay in replying Migelloh,

If you put the new CPU you correctly, and it doesn't work...you can put the original CPU in (correctly of course) and it should kick back up and work without a problem.