That's why (yeah, yeah, I know I'm probably just being a nag) you should always:
A. Make sure the person has a DECENT amount of rep, I trade $ with people who have 50+ rep anywhere, SR, or Skial.
B. Talk to them first, ask questions. I've actually flushed out scammers even before any business was started.
C. I don't mean to be prejudice or anything.. but the scammers usually have very bad typing skills/ bad English. Not always, but it happens quite a bit.
D. If you feel like ANYTHING might be wrong, then don't do it. There are most likely other people that are either selling or buying whatever is wanted in the situation.
Those are my big 4 when doing cash trades. Hopefully you use some parts of this. :)