I just got this Blackberry 7100t off of ebay, and the problem is that when someone talks, i can hear them just fine, but when i talk, nobody can hear a thing. I have to put my bluetooth headset in just to talk. What could be the problem?

It's a very old BlackBerry, bought off of eBay. That could be two problems right there.
Try and get your money back.

Wirelessly posted (Its All About the U!)

Cut your losses