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

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.

any particular transaction?

One thing I learned, is that going thru PayPal, you want an invoice from seller for ____ (goods). Then you get PayPal protection option. If just sending money to a seller, without an invoice, it is like sending cash to friends or family, zero protection.

Choice who pays the small extra protection charge, seller or you? I as buyer pay it, it's me that wants the protection.

Check which credit card has the best protection, perhaps call them first.

Paypal protects the buyer, not the seller.  And Fraud can go either way even with paypal. Audiogon is in bed with paypal and has removed all other forms of payment.  

The best prevention, go pick up the unit personally and pay after you have heard it operate.  In the past (pre-covid 19) that was the majority of the sales I've made.  Even with Covid, there are safe ways to do that.

Speak to the seller on the phone and correspond many times via e-mail to get a feel of the person (buyer and seller).
I prefer to take the risk of accepting check as payment and waiting until my bank tells me the check has cleared before shipping the item.

I don't refuse paypal, however, my listed price does not include the paypal fee.  That way the buyer can pay with cash (pick up) or check.  Paypal fee is added if the buyer wants to use paypay.
If the buyer lives nearby and wants to come hear the unit operate and pay with cash, paypal makes no sense.  

So, don't be married to paypal.

Also, how does one know if the unit for sale isn't stolen?  I need a little background from the seller.  Are you the original owner?  Where did you purchase it from?  One can only go back so far, but if you buy a stolen item, knowingly or not, you have received stolen goods.  I don't know how  on this type of forum, one can 100% protect themselves from this.

Large ticket items require much correspondence, verification of the item, (photo and serial numbers), phone conversations, etc.

Feedback may be an issue, but what if the person has never sold anything?  All of us has at one point had no feedback and sold something for the first time.  It could be an inheritance, death of a loved one, someone gave it to you, etc.

so, to me, correspondence and communication is key.

enjoy