There you are, standing behind the layout operating trains when someone thrusts a model in your direction.
"I'm looking at buying this. Can you see if it works for me".
There is an implied but unspoken, "NOW!"
Or more recently, "Have you got a screwdriver. Can you take this loco off it's base so I can see if the wheels go around before I buy it.", again with an implied, "NOW!"
What the questioner fails to comprehend is that everyone watching the layout has paid to come in and be entertained. Also, layouts are attractions in their own right and not just adjuncts to trade stands. You might only be interested in rummaging around in boxes of junk for bargains but other people are there too.
And traders, please don't point punters at the nearest suitable layout just because you can't be bothered to bring along loco test facilities. I nearly came to blows with one trader who thought that was all we were there for and laughed at the idea we weren't. It's very telling that only "bottom feeder" (table of junk, mis-spelt difficult to read signs, slightly smelly) trade do this and I've not seen him at a show since and I'm not sorry.
The man with the Atlas loco above? Wandered off after handing it over and didn't come back for over an hour. The wheels do go round.