What to do when someone LOOKS like a scammer?

Someone proposed a trade a little bit ago. I didn't recognize the name so I took a look at their profile and trade list. I was very concerned. They haven't traded in over 6 months, their last trade was a negative with the text of "never received my cards," and now they've come back and proposed ~20 trades. I cancelled my trade immediately, but I'm worried that any of the 20 something other traders are getting scammed. What do?