1) If you list an item, contact potential buyers in the order they responded. First to respond= first to get an email. Also, do not make multiple appointments for possible buyers to see your item. This only screws over the ones who come after an appointment when the buyer bought your item.
2) If you sell your item, contact the folks who emailed inquiries to let them know. You know, in case your item was so unbelievably awesome that they're waiting by their computers with baited breath for you to respond.
3) Do not give a time frame for availability and then email that to ALL interested buyers. It creates heart-pounding, unnecessary, UNFAIR stampede on the item.
4) Include pictures in your posting.
5) If an item is dog-chewed, cat-scratched, baby-peed, teenager-vomited, water-stained, food-ridden, or infested with gosh-knows-what, the price needs to be considerably less than what you think it is worth.
6) If you make an appointment with a seller, keep that appointment. And arrive on time.
7) Prices may be negotiable, but don't be stupid. Don't offer $5 for something that's posted for $100. Seriously.
8) Don't sell things out from under a buyer. If I make an appointment at 3pm, I'm going to assume that you don't have an appointment for 2pm. If you do, PLEASE TELL ME when I make the appointment. Making another appointment is still not good behavior, but tellingme is far better than me arriving to find the item SOLD.
9) No, I cannot hold the piece for you. For five days. While you consult your cat as to whether she likes microfiber. No, I do not have the item in another color, or another height or another fabric. What do you think I am, a department store?
10) When responding to an ad, don't respond just with "want." That's dumb. You may be first to email, but I'll bust rule #1 and go to someone with the dignity (and smarts) to type a full sentence, at least.