$20-30 is a very fair price, and they buyer inspected the item beforehand. If he didn't like it, he didn't have to hand you the money and take it home.
Toadstools are pretty hard to kill. Sometimes they look miserable for a while, but rarely die unless they get infected or have too little flow to heal properly and smother.
The truth is, many people in this hobby will try to talk you down both before the sale and then complain after that it was still too much. It doesn't matter how good a deal is; some people are just never happy. If you are straightforward and honest and give your buyers exactly what you promised, then you don't need to do a thing to placate them.