I tend to believe most on this site keep their gear in good shape and these represent fair values used for the most part.
Of course always exceptions.
And sure, sellers always look to put the best spin on a product no matter what or where. Exaggeration is a fact of life, especially when there will be no evidence to the contrary.
Best to deal with those with a good reputation and history of being accurate and reliable whenever possible. I always ask lots of questions before buying used to help gauge. It has worked out pretty well. I have probably participated in a couple of dozen audio related transactions between here and ebay over the last 5 years or so. Only one bad experience on ebay, involving a deadbeat buyer, though that one was a couple of years back and helped tame my enthusiasm for ebay.