Why does that never happen to me?
If it was me, this would happen, I can guarantee it:
I'd pick up the glass and the seller would come over and say, "That's Whitefriars that is, can let you have them for £50. We threw the box away this morning because it was a bit dusty. But it must be rare and valuable Whitefriars, because the box didn't say Whitefriars on it."
Oh, I miss going to antique fairs - I seem to have this conversation at every other stall.