If they want to pick it up in person I make them pay cash on collection and not paypal. I got stung (not bitcoin related) on something where the guy paid via paypal and collected in person and then did a chargeback. Because I had no proof of delivery, I lost.
So, cash on collection only or paypal and they wait for me to send via tracked courier.
I had issues with my bitcoin client recently, after upgrading my RAM. After a memtest (google) it turned out I had a faulty module. The client seems to be quite RAM intensive.