What to do when buyer claims sellers item broken?

The first thing I'd do is call each shipper and notify them of your upcoming claim. Presumably you paid the shippers, so their contracts to perform are with you. Each shipper will create a claim number for everyone's reference. Next ask the buyer to get an estimate for repairs. Then ask the buyer to submit the repair estimate to the shipper, referencing the claim number. If all goes well, the offending shipper will settle with you and refund your shipping fee. Then you can settle with the buyer.

PT,
I believe that it only happened on one of the amps
Did you ask if he/she accidentally connected wrong speaker?
This is the most horrible situation for the tube amp since in this case time delay of fuse might be greater than tolerance to keep such load for tubes.

This indeed is a sensitive issue. Something similar happened to me, where I shipped off a small amp. I believe I got a phone call from the gentleman claiming the unit did not work. Well, the unit was working when it left, and
I reminded him the meters, (which took bulbs) would need the bulbs replaced to be able to see them. Well, that was not the problem. Needless to say I was very upset and depressed and got no sleep that night. I did not hear from him again so I called him, and it turns out he was trying to use some speaker cables with lugs that just weren't making good contact. But, I thought for sure he had blown it up by messing around with it when all it needed were those little bulbs. I don't know how I would have handled it if he had. The main thing I got out of this was the possibility of something like that happening. Sometimes it can be just the stupidest thing, even for someone who's been around this stuff all their life and hooked up a ton of things. So, all I can really say is "Patience, patience" and don't jump to conclusions, just try and work through it. Another time I got a Marantz receiver that I could't get to work no how. Just on a fluke I put a tape in the vcr that was hooked up to it, and "Viola!" sound came out; and I'll never know why it didn't work prior.

I recently shipped a preamp that was checked out by a professional just before shipping. The customer claimed problems of which of course there was no way to verify. Since this was a phono preamp, the customer could have caused issues when setting the unit up for his cartridge. I took the unit back and ate the loss involved in the shipping back and forth. As it turned out a regulator had been shorted out. The customer claimed that one channel was amplifying more than the other and he did internal tests to verify. In reality there are pots for each channel which can be adjusted for such problems. Oh well, not worth losing sleep over.

Tubes amps can blow from connecting without a load (speakers) connected. File a claim with shipper. Ask buyer if a local tech can have a look. Shipper may have dropped unit. You have to get them fixed . Ask buyer if he shorted them or damaged them , see what his response is . Otherwise you have to take them back. I once sold a Revox int. amp to a gentleman. I sound checked it , he picked it up! Next day he says it only plays at very low volume! I had to see whats up so I went to this guys apartment in the furthest away from me downtown part of China Town in NYC . He had a new pair of B&W 802's so I thought wow maybe it blew or got a short. So I check it out and he had the -20 mute button depressed ! Another friend of mine got no sound from his new stereo system , until I released the magic (tape monitor) button . Good luck resolving this mess.