Let's try to avoid that, yeah?
Step 1: request a Pm, if they give some bs excuse (im at work, or whatever else) then call it off, until they can pm onsite.
Step 2: use protection. (Middle man) I'd suggest @sector if you need a free middle man, and @philly if you really wanna be safe (he has a fee though, so don't overlook that)
Step 3: make sure the person you're dealing with is someone you're sure you can trust. If they just joined, and want you to go first, dwc.
Step 4: make sure to secure your newly gained item, you want it putting fort knox to shame.
Step 5: don't be shy to leave a little rep, or a vouch for the other person, as that's polite.
If you get scammed, you have my condolences, but you can try to avoid that in the future.
Thanks,
- shaine