Take that as a very cheap lesson even if for you 7 dollars is a meaningful amount of money you should consider yourself to be lucky there have been people that lost hundreds of bitcoin to scammers that are now worth a fortune, so remember to always use an escrow for any deal, there are some escrows that charge you money but there are others willing to do it for free, just look that the escrow has a good reputation or you could lose your money to them as well.