Is it a rule not to dump your tanks when selling?

Hopefully there is no brown mound.
Same thing happened to us on a fifth wheel from years ago.
Ours was so bad, we had to have the tanks professionally cleaned.
Some people are just pigs. I like the people that put a rig up for sale, and post pics with their junk everywhere, unmade bed, dog cages, etc. Actually just looked at a coach for sale that had pics of a litter box, overflowing with cat poop. :?