One possible way is for them to wait until you've shipped it, then report to PayPal that it was either a bogus charge, or that they never received the PC. Paypal then refunds their money.
One word of advice: If you are selling on criagslist make sure it's local! That's what craigslist is for!
Having said that MAKE SURE the payment you received is paypal funds and NOT a credit card payment through paypal. If they pay with a credit card they can ask for a chargeback overruling any paypal decision. Most credit card companies allow a few no questions asked chargebacks. It's very easy to abuse the system.
If they pay with their paypal balance you can fight it.
I shipped a pc I sold on ebay and the user paid by paypal with a confirmed address in New York. He then reported that the computer was damaged because it had a scratch on it and it was not what he wanted. I said find send it back and I will refund you. Instead he calls his credit card company and makes a fraud claim. Basically my money was taken and I submitted my shipping info and that it was delivered and paypal or ebay did jack shit. I am out $1100 actually my paypal balance just went negative since I had already taken the money out. So I then just said sorry paypal if you don't believe me I guess you are out $1100 not me.