Just a couple of thoughts...
I have bought a lot off Craigslist, although I've never sold.
I collect Apple computers. Even though I can be a hard negotiator, mostly because I buy the kind of stuff I'm looking at often enough that I know what a fair price is. There is no price or rarity guide-Ebay is the best I get plus my own personal record. I actually take upgrades into account, and there are many instances where I've bought a piece specifically BECAUSE of a certain upgrade. That's often reflected in my offer-if it's a $20 computer(no upgrade) with a processor upgrade that I value at $100, my offer is going to be $100. In those case, I'm often valuing the piece only at the value of the upgrade. If I see an ad that is rediculously overpriced, I just skip it unless it's an item good enough to get me excited. The unfortunate thing is that many people see the Apple on it and think it's worth a gold mine.
Even so, I've developed some good relationships. Once I've bought from someone, they often come to me directly rather than posting. They know that I come with cash in hand and ready to deal.
My last vehicle purchase was about 60 miles away. I drove up once to look, then stayed in constant contact. The seller actually dropped the price a few times without my asking, and finally I-once again-went up with cash and bought it. Not too long ago, I was looking at a sort of rare MG that was just up the road from me. It was a 74 1/2 GT, which has the ugly rubber bumpers but there were 1200 and some imported to the US(GT imports ceased after the 75 m/y). Even so, it was attractively priced at his initial offer, but something felt "off" about the clutch when I drove it(the friction point was near the top of the travel). Someone else on the MG forum looked at the car and reported a slipping clutch. The seller eventually texted me and offered it to me for $2500($1K off his initial price) but I knew I had a transmission swap in my future with the roadster and didn't think I could handle two engine pulls within a couple of week 
. There's also the fact that by the time I pay $150 for the new clutch, but by the time I cover all the "might as wells" I'm at $500 or better.
A year or so ago, I saw a for-sale ad posted in the barber shop. My barber really talked up the car and said he'd seen the car. I called the guy, then went over and drove the car that same day. Unfortunately, I hadn't done my research beforehand and realized he was over double book on the car(14K vs. 6K). I called him on Monday(I'd looked at the car on Saturday), explained the situation and just politely said "here's where I'm at" and offered 1K over book($7K). To me, it wasn't worth getting into an argument where I probably still wouldn't get to a price where I was happy. He countered with a condition and mileage argument, which were good arguments but not $7K arguments . Again, very politely I said I was going to have to pass and wished him the best of luck on selling. I saw the seller at the barber shop a month ago, and he told me that he'd just decided to keep the car because it was "really nice and a lot of fun." I agreed with him fully on that, and we just had a nice chat in general.
. There's also the fact that by the time I pay $150 for the new clutch, but by the time I cover all the "might as wells" I'm at $500 or better.
A year or so ago, I saw a for-sale ad posted in the barber shop. My barber really talked up the car and said he'd seen the car. I called the guy, then went over and drove the car that same day. Unfortunately, I hadn't done my research beforehand and realized he was over double book on the car(14K vs. 6K). I called him on Monday(I'd looked at the car on Saturday), explained the situation and just politely said "here's where I'm at" and offered 1K over book($7K). To me, it wasn't worth getting into an argument where I probably still wouldn't get to a price where I was happy. He countered with a condition and mileage argument, which were good arguments but not $7K arguments . Again, very politely I said I was going to have to pass and wished him the best of luck on selling. I saw the seller at the barber shop a month ago, and he told me that he'd just decided to keep the car because it was "really nice and a lot of fun." I agreed with him fully on that, and we just had a nice chat in general.
