When logged into the boards, click "Profile" under "My Content" on the left sidebar of the screen. This will show you a list of "recent messages" you have posted on the right hand side of the Profile screen.

If you cannot find a post you made, it is possible it was removed by moderation for a violation of the Boards Usage Policy. If this is the case, you should have received an email directly to the email address you have registered your eBay account with explaining why your post was removed.