How to not be an asshole on craigslist
1) If you inquire about something and then change your mind or decide it won't work once the seller provides you with the details you requested, don't just delete the e-mail without replying. Seriously, take the 30 seconds to write back and say it's not quite what you wanted and to thank them for their time. You wouldn't walk into a shop and ask to see some items in various colors, then just walk out without even saying a word. Or perhaps you would, and that's the problem.
2) If you've shown interest in an item and you ask the person where they live, don't then reply and say, "oh, I don't know where that is, so I'm all set, thanks." If you can't use google maps or ask someone for directions, you have no business posting on craigslist and scouting around the city to try to get one-of-a-kind items and good deals. Likewise if you realize my neighborhood isn't posh enough for you. It's right by a major T station, has plenty of on-street parking, and I've already specified the general area in my original ad. Please, go take your city-fearing self to a Jordan's or something. Craigslist isn't for people like you. And furthermore, when you decide not to venture into the city to check out my table, you're not "all set," because you still don't have a table. Grow a pair and use your I-statements and tell me you've changed your mind or are going to have to decline or whatever. The only place where it's acceptable to say "all set" is in a restaurant or at a cash register, and even then it's not always the phrase you want.
3) When you come over to someone's house to look at things, don't freak out and tell them you're allergic to cats and lecture them as if they had just brought cats into your house without asking. It's not at all uncommon for a North American household to have cats or dogs, so you might want to ask people before visiting their home. This especially goes for the person who wanted to buy an all-wooden piece of furniture and said that I should have stated in the ad that it was "contaminated." Thanks for the feedback, and next time I will be sure to anticipate that my buyer might be someone who will be deathly allergic to wooden furniture that has been near cats and has since been wiped down.
4) Don't try to negotiate the price without at least some sort of background as to why it probably isn't worth as much, or acknowledgement that you realize you're not offering much and you're happy to wait and see if there aren't any better offers. I did sell a mandolin to a very nice person named Jackie, who explained to me that it needed a few repairs and how much these would cost, and who was apologetic and made it clear that I shouldn't feel obligated to listen to her. This was very upfront and helpful, and I was happy to sell it to her and felt confident that she knew what she was talking about. Others, however, have been less appropriate with their offers. Like the person who sent me a link on Amazon to a product that wasn't remotely similar and said my item was overpriced because they could buy a completely different item new on Amazon for just a little bit more than I was asking (plus shipping that cost as much as the item). Or the person who asked if they could buy my almost-new huge suitcase for $15 instead of $20, and would I mind dropping it off in Providence.
5) When you're getting a really good deal on something, don't decide that you're only able to pick it up at one specific time on one specific day. Shopping opportunities do exist for people with such important obligations and inflexible schedules; they're called stores. For four times as much as I want for my bookshelf, you can get one exactly when and where you prefer.
6) Respect people's time. If you need an item to fit in a certain space, and the dimensions are included in the ad, measure your space and see if it will fit before coming to look at the item and saying, "oh, it's much bigger than it looked in the pictures, and definitely won't fit in our room" and being surprised that a table that seats four is larger than 600x800 pixels.
7) Don't offer to mail me a cashier's check. Just don't.