It's a matter of trust. Check the seller's history and reputation on their profile. If still unsure, you could ask for references to previous buyers.I’m gonna make my first purchase through classifieds today. What’s the usual safest transaction method; PayPal?
Paypal F&F (friends and family) is popular because there are no fees. And also no PayPal buyer protection. If you want the buyer protection, then typically you would cover the fees.