Some basic tips off the top of my head:
- Use chargeback free payment options like Western Union, Money Gram, Liberty Reserve, Player Auctions, Moneypack, etc
- Look at the person's vouch count, post count, etc
- If they're overpaying and really willing to go first its likely a scam
- You can ask for people's IDs to prove their ownership of their paypal account (People can still use proxies to make it seem like they're hacked)
- I'd suggest accepting payments as services with a note like "Payment for a service already completed" rather than gifts.
Other than the first option you can't completely prevent chargebacks, but after a while you learn how to better protect yourself.