Adventure,
In theory, you're right--the problem should not be happening. However, something you cannot see may be causing the blockage.
If it were me, I would remove the seat and then the main filler neck hose at the tank and look inside with a flashlight. Same for the sender unit--remove and look inside. Possibly use a bent coat hanger to feel around inside. Also run the coat hanger into the tank vent line and down the main neck as well. Then if nothing, drain out the gas and remove the tank for a better view.
Since the seller already mentioned the problem, someone could have purposely or accidentally put something down the fill neck--a sponge, an old sock, some tampons from a jealous girlfriend; perhaps the tank was coated inside, or surgery done to the tank that's only visible once you see the bottom of the tank.
Somewhat related: I noticed years ago that one of the toilets in my new-to-me house was slow to flush, plunged it repeatedly--no luck. Ran a snake with a coiled-end into it and pulled out a ladies eyelash curler that would block toilet paper and "stuff" until things dissolved. Appears one of the previous owner's little kids liked to flush Mommy's stuff down the toilet for kicks.
Anyway, good luck with your adventure.
