I have 300+ transactions with Paypal and after many disputes going against my favor when I was in the right here are my two cents.
I've had many F&F get charged back.
I've had many G&S get charged back.
INVOICES!
Use invoices with a breakdown on what is happening (IE: your profile on the site, their profile on the site and how the delivery will happen) and that the delivery of the goods being non-refundable if the invoice is paid. Collect screenshots once delivery is made in case any messages are altered. By the terms you set in the invoices and the screenshots you collect, this has shown to be in my favor on every single transaction that was disputed, when both F&F and G&S were mostly rejected. I may have only won like 10% of those disputes.
If you're trading in 3rd party apps where usernames can be changed, messages edited, and other manipulation happening you will likely be scammed, this is why most business should happen through the site for both parties protection, and if possible to use trade guardian for further protection.