Bold 9900 only works while on charge ?

I recently purchased a bold 9900, but as soon as I unplug from charge it dies.. Any clue what the problem could be ? I have tried a different battery

I would say the phone is defective. Probably where it connects to the battery. Tried replacing the phone? Defects like that are usually covered under warranty.

The phone was purchased 2nd hand from eBay :-(

Could be just a bad (or heavily used) battery. Have you tried a fresh one to see if that's the issue?

EDIT: sorry I missed that you had already tried another battery.

Then it sounds like a defective phone. Can you contact the seller to see if they would take it back or refund you your money? Hopefully it wasn't sold to you with the knowledge it was defective.
Sent from my BlackBerry 9900 using Tapatalk

Battery might not be getting good contact. Fold something like a thin piece of cardboard and wedge in the top gap to tighten the fit. Also examine the pins in the phone and the pads on the battery. make sure the pins are level and straight and that the pads on the battery are clean. If not lightly scrape the pads with an edge of a knife tip to clean and expose the bare metal. Lightly.