This is what I would do:
1. If the bait sold for less than $20, apoligize and refund his money. Let him just keep it.
2. If the bait sold for more than $20, apologize and ask him to send the bait back. Then refund his money.
3. Toss the bait in the water. If it is defeective, toss it out. If it is good, toss it back on e-bay.
Keeping a good rapport and reputation is very important. A twenty dolllar bill is not going to make or break you. I have learned many times over the years, that allot of the customer that break my balls turn out to be some of the best customers I have.