You need to include the location of where your notices are being hung up. For example, don't make a post stating your character wants to buy an item, with no information as to where the notice is placed or how to contact your character.
Avoid going OOC in any in-character forum. This includes merchant posters, don't list the OOC stats of items. Instead, give an IC description of the item. Listing OOC time and time zone of an event is acceptable.
Do not run auctions, negotiate or otherwise discuss details of a trade on the forums.
Forum correspondence involving trade should be limited to agreeing on a time and place to meet up in game. Try and place notices in-game as well as posting them on the forum. Don't use @mark, those disappear on area re-initializations. Instead, use a pen and write on a paper object using @write.
If the notice is at all antagonistic, we expect copies to be placed in-game exactly where they are described as being placed in the forum post. This includes any libel, posts calling other people dangerous, posts attacking a religion, and so forth. If you need to bump a thread, keep it in-character with a [new posters are hung up] post or something similar. If you bump a thread more than once a day without adding new information to your post, it is considered spam. The best way to advertise for an event is to have in-game posters, an in-game poster won't get lost under other posts and draws attention to itself by being an object in-game.