i bought a used iphone from a friend and have found out that he was making payments on it, but didnt pay it off yet. Can i use it
Good question. I've never heard that one before. Most likely they will bill him for the outstanding balance. Whether or not they will activate it for you is a question you should ask the carrier.
I have heard that GSM carriers will now blacklist the IMEI of such phones, like CDMA carriers have always done with ESNs. The phone might work for a while, but if the bill is not paid, it may eventually be cut-off, no matter which new carrier you are using it with.