I bought an unlocked iphone back in the usa and i just got home in Romania but it is not working with the carrier?

Did you activate it on a US carrier? If so, the carrier you activated it on is the one it's locked to.

If you did NOT activate it on a US carrier, it is still tied to the US flex policy for activation. You can't get it unlocked until you first activate it on a US carrier, after which you must meet that carriers requirements for unlocking.

Some US carriers have minimum time periods in which the device must be active on their network. Some will not unlock it unless it is currently active on their network. Depending on the carrier used to activate it, minimum service periods can range anywhere from 40 days to one year.

Where, exactly, did you buy the phone? If you purchased it from a retailer other than Apple (e.g. Best Buy, Target, Walmart, etc.) then you did NOT purchase an unlocked phone. The only source for unlocked iPhones in the US is directly from Apple.

They lied.

Best Buy does not sell unlocked phones. The phone you bought can only be activated on a US carrier. Once activated on a US carrier, it will lock to that carrier. At that point, you must contact the carrier it's locked to to request an unlock.

Try http://osxdaily.com/2015/02/08/check-what-cellular-carrier-iphone-uses/

Settings > General > About > Carrier

It would be far easier for him to return the phone to Best Buy and buy one unlocked from Apple.

Do you have any friends in the US? You could ship the phone to a friend and let them handle the return.

True, but that is impossible. It must be done where it was purchased.