If you're that concerned, you have a couple of options that come to mind.
1) You could use a service like bit-pay.com to generate a payment button that is set at a fixed USD price. They would take 1%, and you would have to wait until they made payment to you, so the risk is still there. This is a good solution if you are selling items over a long period of time, like on a website.
2) You could open an account at bitcoinica.com and 'short' the amount of bitcoins that you're asking for in the trade. You would most incur a small loss with this strategy due to the 'spread' on bitcoinica. This is a good strategy if you're dealing with extremely large bitcoin trades. Not advised for the newbie.
3) You could give the person you're trading with a deadline to make payment. You'll have to wait for confirmations to your account no matter what you do, which will open you up to a little risk. This is the most common and least risky way for small trades on the forum.