Fraud, what do you look for to spot a risk?

To go on a limb and actually provide an answer,
first and foremost, look at seller feedback and how long they've been selling.

Feedback and PayPal.
No worries

+1 on fuzztone’s formula. I also use the ad as an indicator. Does the seller offer specific details that accurately describe the product, AND do the photos confirm the description. For instance, if an amp is rated 9, but you see dings or scratches in the photo, beware, or ask the seller some direct questions. (Not every seller responds well to this approach, which may also be useful information.) The good news is that honest, trustworthy sellers of surplus audio goods far outnumber those who may be somewhat deceptive. 

Also, if you contact a seller and he/she isn't forthcoming with information, then back away.
FWIW, I was a newbie buyer here on Audiogon only a few years ago.
When I bought locally, almost all sellers were agreeable to my stopping by for an audition.
Distant sellers would allow me to call them.
In every case, I found the people here to be honest, and often helpful with problem/questions I needed answered.
YMMV
Bob

If they refuse PayPal stay away. There is your possible fraud. If they want you to pay the fees it better be factored into the price. Or offer less.
You should not have to worry here. I've never encountered any problems here. 
The big bad internet, beware.