My own personal take is pretty much what has been said by others - I firmly believe that you reap what you sow in this life, without wanting to get all mystical - and if I get a decent deal on something I would generally rather pass that on to someone else than profit from it when the time comes to move it on. In the, perhaps naive hope that I will get treated the same. If I have spent a lot of time and money on something getting it fixed, that is a different matter.
But that is me. We all have different values and go about our daily business in different ways. What is acceptable to some is sharp practice to others - it is like this in the rest of the world, no reason why bass players should be any different. Ultimately no-one has a gun to your head when you are looking to buy something - caveat emptor and all that - so do the necessary research and make your own judgements. I bought a pretty dud bass from someone on here once but I genuinely don't think he knew it so I didn't leave any negative feedback. My own tough luck.