Ray and sullim are both correct. Since the 2002 redesign, all game-issued Bills jerseys (including the throwbacks) have sewn numbers, and the "authentics" are based on those designs. If the numbers are screened, it's a replica.
On the other hand, if you're talking about a Kelly-era jersey, sewn numbers are a dead giveaway that the shirt isn't real. Back then, the Bills were one of the few teams in the league to use screened numbers on their game jerseys.
If it's listed as an authentic on eBay for a significant markdown from the suggested retail price, it's almost certainly a knockoff. If you're all right with that idea, go for it. The only current "authentics" I see on there are a couple of different Lynch jerseys, and they all look bogus to me. One easy way to tell: look at how the shoulders are cut.
Real jerseys will be wider at the top, tapering to where the dazzle cloth used for the shoulders meets the mesh of the body. The shoulder-to-sleeve seam on the
counterfeits typically runs straight up and down, giving the shirt a "boxy" look. The sleeve cuffs on the real ones are tighter, too.
And if you buy from
this guy, you are guaranteed to get an authentic autograph on a "custom made authentic style" (read: unlicensed) shirt. You want to pay for that? Caveat emptor.