Yeah it would have to come across sincere. They'd have to make me believe that they knew they were being cheeky, and knew I was doing them a big favour.
When I was a teenager I used to work in a supermarket. Customers who asked me politely/nicely if we had stuff in the warehouse, I would look...
I read that and the liked the "f**** off" response. But actually on reflection, it would probably depend on how they asked me. If someone acknowledged that it was really cheeky, and that i would be massively helping them out, I might just let them have the table.