In my area people are pretty honest about the items they are selling, or maybe I just don't look at items where the seller is pushing too hard, either way I've had pretty good luck.
For selling my stuff I take good pictures and plenty of them, then list what it is and anything I know to be wrong with it. I stay away from talking it up. I figure if they want it they'll sell themselves on it. I also sell cash only and as is.
When buying I stay away from trash talking. Sometimes I'll make an offer, but not always, kind of get a feel for if they priced it with room for offers or not. Most of the ads around here say a price and then OBO or make offers or something like that.
A machine I bought I drove two hours to look at it, his ad said OBO, he was asking $3200 and I offered $3000. He said sure and then took me to lunch after we got loaded. We were both happy with the deal and walked away friends, that to me is a good transaction.
One used machine I was buying from a lady and it broke down as I was loading it, threw a rod. She offered me my money back. I told her "No, I bought it as is and knew there was a bad knock in the motor, I don't consider you responsible for this."