In 1980 I bought a 1965 Gibson L50 (or is it LG 50?) - a guitar the same size and shape as my '55 Martin 00018 - from a recovering drug addict who really needed the cash and asked just $125 for it. I'm sure it was worth several times that much, even back then, but I could not have paid much more for a second guitar back then, no matter how valuable the instrument.   The seller was happy (and might well have hocked it for much less had I not bought it) and I was happy to have a less-valuable (than the Martin) second guitar to carry around to various places.
About 2003 or so, having had a bad case of G.A.S. in 1998 and having 11 different guitars in my home, and needing cash, I sold several of the guitars at a yard sale.   I sold an older, Japanese-built Sigma dreadnaught for $100, having paid $50 for it at a thrift store a while back.   I sold my Martin backpacker for about the same - about 60% of the sale price I'd paid for it. And I sold the Gibson for either $290 or $300 - again, far less than its fair market value - but to a musician who really needed the guitar and would really appreciate it.   
I think everyone was happy, all around.
And the Gibson came back into my life recently, when a neighbor family stopped me on the street and said they had my old Gibson and the teenage daughter had adopted it as her primary instrument.   The dad is a multi-instrument musician and sound engineer, and it was a friend of his who had bought the Gibson but over the years hadn't played it a lot; my neighbor had been lusting after it and the friend had given it to him a year or so ago. Now it has a very appreciative budding young talent playing it, and I've made new friends and a valuable music contact indirectly from having bought and sold that guitar, both times well below the value of the instrument. The value of an instrument sale isn't necessarily measured in dollars.