I don't see anything in the PMs posted that says "as is" or in any way says there is no warranty or guarantee. There was an explicit statement that it was working as it should, and it wasn't even couched in "I think" or "to the best of my knowledge".
The buyer is the credit card company's customer, they are going to side with him over you. You could file a small claims suit against him and get a more objective ruling but I think you would still lose.
You should request the buyer to send you $41, which is the difference between the $113 he agreed to pay for a working unit and the $72 he has actually paid (to the shop) to make it a working unit. The buyer is actually better off than if the unit had worked in the first place, since he now has a unit that has been inspected by a shop tech and can be more comfortable that it is in proper condition.
The buyer is the credit card company's customer, they are going to side with him over you. You could file a small claims suit against him and get a more objective ruling but I think you would still lose.
You should request the buyer to send you $41, which is the difference between the $113 he agreed to pay for a working unit and the $72 he has actually paid (to the shop) to make it a working unit. The buyer is actually better off than if the unit had worked in the first place, since he now has a unit that has been inspected by a shop tech and can be more comfortable that it is in proper condition.