IMHO, I wouldn't consider someone who pays money to buy a table at a train show, particularly where admission is being charged, to sell a number of train related items to be either a "private sale" or a "non-business" sale situation. I would classify that person as a vendor (I know - more definitions) and that's probably why a sales tax number is required.
I guess New York State agrees with you
I think that's very different from a homeowner's isolated garage sale. I don't know any homeowner that holds a garage sale every year. They must be buying more stuff than me !
It is common in my neck of the woods.
Some folks pick all year for their annual garage sale.
NYS probably would want their due in this situation too, but it's harder to track/enforce.