If I can buy a iphone 3 from somebody, can i use it with T-mobile prepaid?

The former owner of the phone who is selling it says that they used it with T mobile on the prepaid and on the 2year plan. Would it still be possible for me to use it on the prepaid plan if I got a plan of my own through t-mobile?

Yes, that will work.

As long as the phone is not locked, you should be able to use any prepaid phone provider.

all you need to do is replace the sim card. I have a locked ATT iphone 4 and i need to jailbreak it in order to use Tmobile prepaid card