12. Thread dumping. Threads are for people to sell their items, not for you to inform them that you've seen the same item cheaper elsewhere, or that another product is better and cheaper.

13. Complaining about lowballers. It's a fact of life that people are going to offer prices far lower than what you're asking. If this happens to you, a simple "no thanks" is all that's required, not a public lambasting of said buyer.