Great amp, crashed and burned after 30 hours... Buyer protection?

Sorry, Just take the cover off. If my buddy had taken off the cover when he first bought the amp, he would have seen the some of the work and known right away it had been tampered with... your nervous?! Imagine me with the horror story to go further in the hobby!!
I think at the end of the day, my buddy was really choked with the sellers attitude.... because up to than he was very happy with every deal, on all the websites... one rotten apple in the barrel I suppose.. what we call the "A-hole" factor in my businesses... (will that be edited out?)

When buying used over the internet there are no fail safe ways to guarantee that you won't ever get scammed.  The risk comes with the territory.  Use caution and common sense.

Did your buddy happen to deal with a Audiogon member or someone from another site?

It may be off-base to blame the seller 
given the sudden and catastrophic failure of the gear, it may be a crap build equipment issue.

i had three (3) sudden .... And mean unforcasted and SUDDEN .... Catastrophic failures on the $4k top-of-line AVR and blu-Ray player.
with the receiver , it was fine and next use two days later .... Kaboom! New motherboard replacement and gone for 5 weeks 
warranty coverage ... Okay....but. Something amiss. 11months later was an instant repeat. Good thing it had a three yr warranty 

the Bluray player ( a $1500 unit) crapped out catastrophically suddenly in mid-movie . It was 27 months old and three months out warranty so I was SOL with repair costs exceeding its FMV.

I sold the AVR and replaced the BR with a quality build Euro unit...and Euro / USA quality build separates 

the moral of the tale:

- the  expensive and shown-to-be-unreliable Chi-fi gear I had was crap build POS gear ...full stop

- mfg OEM warranty through a distributor matters. Too many have only a 1 year .... Caveat emptor. Most used gear won't have it and the bargain price reflects that.

- your seller may have sold used gear on a proper belief that it was out-of-warranty but all good ON DELIVERY with no knowledge of any issues. The risk is entirely yours