while i do feel bad for the fish, the bottom line is, you are trying to make a business transaction. the fish are already there, it is not your responsibility to care for them, unless you offer to put them in your tank.
lets say you were selling a 2 br house. if a man came to buy it and you thought it was too small for his 12 kids, their 4 kids, his sickly mother and his wives parents, are you really going to refuse his money? is it really your problem how many people he crams into the house?
i am not saying it is right to house oscars in a 30g. i would never do it, and i do not believe that anyone else on here would. but all we can do is try to take care of our own. we cannot afford to mix business with personal desires. it just doesnt put food on the table or in your tanks. you did just fine selling him the tank.
as pt barnum said "there is a sucker born every minute. and i want a dollar from each of them."