Poppy,
I happen to sell Celtics tickets (and many people on this site might know from the tickets forum) so I think that I know a little on this topic.
Season ticket holders can "print" tickets from the celtics website as many times as they want. Each time that they print, the previous tickets become invalidated.
Were your tickets hard tickets or printed out tickets (on regular paper)?
I know this guy says that he's not a season ticket holder, but there are 2 ways that he can still mess you over:
1. If they were pdf tickets, then he can just resell or use those same tickets and just tell the person to get there before you do....once those barcodes are scanned in, any other copy of the tickets will be rejected at the turnstile.
2. If he really is a STH or he bought it from a STH, he can get the reprinted tickets and invalidate yours.
My advice would be to go to the game a little bit early and try to get in with the tickets that you have. If you get rejected, you can always find a scalper on the street. Tickets will be much cheaper right before the game, especially with the way the Cavs blew them out last game...
If you don't like buying from actual "scalpers" on the street, you could go to a nearby internet cafe and go online to stubhub. They should have tickets up until tipoff and again tickets will be pretty cheap. They will give you directions to their nearby location where you can pick up the tickets (its run by stubhub). Or you could buy from the Celtics ticketexchange, but you would have to be able to print the tickets out yourself.
But my best advice would be to just buy from me next time and you won't have all of these problems
