What has been your costliest mistake in this hobby?

If this is a true story, and I doubt it.  I don't know how you can smile every evening at 6:30.  I honestly don't understand it. 
You admit in a public forum that you are a liar and in this case also a cheat.  It is one thing to say well I don't know if and how well they work, bad tubes, and the cost of any repair etc, I'll offer you $250 for the pair.  He can accept or reject.  Only looker or not, you knew what the were,  $30,000  + monoblocks (new) and you told the kid they were “organ amps” from an old Hammond electronic organ... not worth much."  Now he should have done his homework there is no excusing that but you lied and therefor cheated him.
I and most other "real" men who know who and what they are, including their flaws, work on their flaws to improve themselves and have character consider what you did disgusting!  A "real" man has character, honesty and integrity. They can be trusted.  You have none of those qualifications.  Maybe your wife is mad at herself for marrying someone who has no character.Karma has a way.....maybe some day you'll get home early and find your wife blo...g that 23 yr old kid.Have a nice day.

Bought a Signature 9 SE CD(and DVD) player for nearly $4,000 25 years ago. Used it for about 50 hours. Just sold it for $500 on Audiogon. It had much worse sound than my Kyocera 300 and 400 CD players from 1984 (sound like a Mac 30 amp, gorgeous mids, light bass, not overally detailed). It sounded sluggish and mediocre at the frequency ends. It had a difficult dual function remote, tough to use. Another big mistake was selling my Acoustat 2+2s and replacing them with Martin Logan Monolith IIIs. For the little money I got for the Acoustats, I should have kept them and just upgraded my system more (this was back in the 1990s).

@stereo5  I've read of your problems with the VR33s on AudioCircle.  It could be any number of problems but they didn't work in your room.   Nearly all of the owners wrote that it could have been your room/floor/acoustics, cabling, equipment, etc.  Something didn't jive for you.  I own speakers some people hate.  The reason is they are trying to play my big speakers in small rooms-it doesn't work.   

Agree with Kenny, starting hobby....  actually not too many mistakes except for  buying a used VTL..... that was a costly mistake.....   some "lifetime transferable warranty" that proved to be.

My cat enjoyed the comforts of the heat radiating from my Mark Levinson amp.  Unfortunately he apparently couldn’t control his bladder and the result fried the circuit board. I got about $100 for the salvage parts.