My suggestion: Change the icon on the left of the thread where it appears in the forums when you have posted in the thread (the little "envelope" icon). This could have a tick or a mark on it to make it obvious that you have posted in the thread. Here is an example taken from another forum using this:

(indicating that I have posted in the thread)
1. You sometimes forget you've added your opinion to a post. This would help to prevent accidental double posts / spam.
2. It would help you to see when you have made an offer in a trading thread.
3. It is used by lots of other forums.
Please vote and leave constructive criticism, especially if you vote "no"!!!