Steam Trades I'd recommend over TF2 Outpost if you're wary about being scammed as there's a clear positive/negative feedback system (e.g. someone with a +150/+0 rating has been given the tick of approval 150 times).
You can't really be scammed in tftoutpost though. If the seller reverses payment on something you traded for steam will reverse the trade. Just make sure you are getting what you asked for.