I would do this:
Offer the buyer to send it back to me. I would return the caliper with the piston removed, and I would pay both shipping.
If I can't manage to get the piston out, I would give a full refund.
I could not really do much more to satisfy a customer.
Good advise, I'll use it, or part of it, thanks!
Little update.
Got it back today. Looks like it wasn't touched at all ( I hate liars! ). I got out the grease gun, 10 minutes later its out. Sent the buyer an email. Told him about the "good news" and that I'll be sending it back to him and wont be charging him shipping, labor or materials. Told him to find a new shop. I'll ship it back for free, but I wont pay for his shipping as he lied to try for a refund. I looked up his name, he's an adult and should know better.