There is some common sense stuff you can do quietly if you are thinking about trading with someone (be it coupons, gift cards or what not)

Use the search feature on the forum . On most forums you can search by nick and see someone's posts. Failing that google their ID. Look at where the person posts, how they contribute. Then use your common sense filter (the one in your brain).

I'm not saying don't trade with newbies. But why trade with folks who make negative posts, who only post about stuff they want? Did the person make a real introduction if they are new? Are they participating in the board or are they only popping up to ask for free stuff? Do you like their tone? Do they have attitude? What is the tone of their PM?

If your little voice speaks up then don't finalize the trade with them.

Oh and if the trade is too good to be true it probably is, if the person is too eager that can be a flag too!

Common sense. Based on what you've seen of them would you meet them at (insert name of your favourite place to go for coffee) for coffee? If you have no interest in meeting them, don't trade with them.

I don't do many trades online. But yes, I read one too many posts about folks bitching about trades while sipping my morning coffee.