I often do wonder about that, especially when I see the same bass changing hands 3 or 4 times within a short period of time. It's awesome that we can find such listings on here! For example:
March 2017:
SOLD - FINAL PRICE DROP - Alleva Coppolo LM5 Olympic White - Stunner
Nov 2017:
SOLD - Alleva Coppolo LM5
Aug 2018:
SOLD - Alleva Coppolo LM 5 Deluxe Super light (7.2lbs)
Ask me in a few days whether these three guys got rid of it because it was a lemon or for some other reason.
Obviously, I'm expecting it was the latter.
I have "taken a chance" and bought several basses on here that had changed hands a few cases. In a few cases, the bass was just absolutely STELLAR and I found myself thinking that the seller must have been either NUTS or DESPERATE to let it go. But in most other cases, the bass was "just OK" or less when I received it, but then after I adjusted the truss rod and saddles, and perhaps changed the strings, it was instantly transformed into the awesome instrument that I was hoping it would be. In ZERO cases (out of now about 50 purchases) did I receive a total lemon. In one case, the owner had over-filed the nut under the E string so it buzzes badly at the first "fret" position. So the seller gave me a $100 credit toward getting the nut replaced. I still have to do that so that I can play the bass. And in another case, the $4900 bass arrived with undisclosed scratches and a chip, and the seller gave me a $350 credit, which I thought was reasonable enough. In the other 48 or so cases, the bass was perfect except for needing setup work. I continue to be amazed by the number of bass owners who clearly do not know how to set up their bass. In several cases, I found myself feeling guilty and sorry for buying a bass away from someone who probably thought they were selling a lemon (because when it arrived it played like crap), when after just 15 minutes with the wrenches it became totally awesome.