Unless you value your item at $3, do not agree to $3 trades!
You're asking to be scammed by agreeing to a $3 trade. If you are scammed, SidelineSwap can only reimburse you the full cost of your order, nothing more.
Insure your trade. This means agreeing to buy each other's items at their full value. When your side of the trade has arrived at the buyer's address, you get your money back and you're protected.