I hear yah, i do love a discount myself!
Model shows will typically hand out a flier for vendors (private or otherwise) stating that it is their responsibility to collect sales tax and pay the government when the show is over.
I doubt many "closet clean out" sellers even bother at local shows, but a major retailer would be foolish not to collect.....no fun looking over your shoulder. (there probably is even a min amount you could sell and not report it.)
One could argue the man power it would take to police each model show or reptile convention etc to catch these people would be enormous for little gain though.
It's also a time saver too at the shows (to include sales tax), who wants to walk up to the vendor and watch him tap on his calculator when he could just included it in the sticker and take hit later.