I'm sorry, I do not fully know the answer to this question but I will tell you about as much as I do.
Anyone can fake a stolen PP by getting on a proxy and then accessing their account (different IP). It is safest to receive money from people with Bank Verified paypals.
TBH you just need to trust people, or start using other sites such as WU/Playerauctions. What do you plan on selling? Remember that paypal is almost ALWAYS recoverable, so only accept paypals from members you deem trusted.
For examle, a new account might scam you with anything, $5 even.
If someone has taken time on they're sythe account to help the community, it will take them more money to start thinking about scamming people.
A general rule of thumb is to never trust people with more than their Sythe.org account is "worth."