In the 200 series, remove the access cover to the fuel sending unit. More difficult in the wagons than the sedans. Other models I'm not sure how difficult.

There will be some dirt and small particles if not in a flood. A layer of dried mud if in a flood. The difference will be very evident. The one place that cannot be cleaned without dropping the tank is the top of the fuel tank. And if they try to spray across the top of the tank, the dirt and water ends up in the sender access recesses anyway.

This is one thing you want to do someplace other than in front of the seller.

Having the car checked out by your mechanic should allow you to drive the car someplace out of their view.

Duane