I had a similar sort of situation selling my old 90 and 65 gallon tanks. only this time the issue was weight.
my tanks were on a stacked metal stand, so both tanks were on one stand. which means you have the full weight of a 150 gallon tank (+ gravel) in only the area of a normal 75 gallon. you can't put that kind of concentrated weight on a normal residental wooden floor without reinforcement. I happened to ask him where he was putting the tank and he said over his garage. I explained to him that I could NOT be responsible for problems with weight, I am not an engineer etc.
anyways, he bought the setup, but put it back up for sale the next day on the same ads I used for the same price. I guess I scared him off, which was better in the end I guess...