I guess this is true for most online second-hand sales sites. We don't have Craigslist over here, but other comparable sites. I too have the same experience as you.

First it's hard to have them agree to a meet. Then, when they show up, they start haggling. I really hate that because by them you've already agreed on the price, but they start haggling nonetheless since they are there anyway. I find this extremely disrespectful and rude. The last time this happened (again) I finally had enough. I just said "no" and turned around. They guy then showed me a list of other sellers of similar stuff, saying there where enough others like me (how's that for respect?). I wished him the best of luck trying to pressure the others and asked him if it was worth his gas money and his time just to haggle a few euros from the price. He then very reluctantly agreed and paid the agreed upon price. I should have been happy with that I guess, but instead I was really disgusted.

I've had people calling me names because I didn't respond to them within the hour. I've had people texting me threatning texts (I used to provide my cell phone number with the ads - no more), I've had people actually ordering me around ("send me photos of the item NOW" - just like that but usually with some spelling mistakes), if had a lot of people not bothering showing up upon the agreed spot etc...

So since then, when I'm selling something and I get a response, I try really hard to 'read' what person it is before agreeing on a sale. I try googling his name, looking for user feedback about the person, checking the grammar of the mail -no blatant spelling mistakes etc...
This way I've been able to weed out the most rude and disrespectful people. I hardly get burned by those any more now. If I'm not sure about a person, I don't sell to him. I know this might mean that occasionally I'm wrong about a person, but that still beats the frustration of having to go trough the whole process of haggling and rudeness if I ended up being right.