My biggest gripes lately are with buying and selling!
Even though I've only completed a few trades through ASC, I've sold dozens of items on Kijiji and feel like I've established pretty good trading etiquette.
1. Follow the darn trading rules, don't delete the price after its sold.
2. Reasonable pricing. Sure... maybe you got screwed and paid more for something that's now selling for a lot less. Did you get use out of it? What was your use worth? Most people aren't dumb; price your gear reasonably, it will sell faster and you can spend less time worry about bumping that 4-month-old thread.
3. More often then not, I get super annoyed by "Pick-up only". If your house is in the middle of no-where, or in some remote part of town with minimal transit access, you'll have a harder time closing a sale on that $15 pair of iron sights. I always try to provide alternative meet up points. Sure you can come to my house... but I also play airsoft on X day, or work downtown on Y day... etc.
4. First come first serve...wut? If someone says they're coming to get your item, honour their word until the time they agreed to come; don't make them scramble to your house only to find out you sold it to some other guy. Tell the first person they're first, and the second person they're second. If you can post an ad, you can reply to the people who respond.
5. Haggle, but don't haggle. It's alright to ask if the seller will accept less for an item, but do it beforehand, not in-person. I've had people show up to my door with wicked excuses as to why they want to pay me $20 for all my items, versus $20 for each item as advertised etc...
*Busy signing for his packages*