Silver -
It's true...if the seller pulls the money from the account, the buyer cannot get it back.
From the other side of it: My husband sold a laptop on ebay using paypal. He received the money from the buyer in his paypal account, so he boxed the laptop up and dropped it off at our local FedEx place. He then sent an email to the buyer with the tracking #. That same evening, he got an email from either ebay or paypal (I forget which) telling him that that the buyer was under investigation for something. My huband quickly checked is paypal account, and sure enough...the money the guy had sent had been pulled out! AFTER he got the tracking #! My husband immediately called the FedEx place and the driver had not yet picked up the package for shipment, so he was able to get it back. He would have just sent a laptop to some guy for free.
Now, when he sells a big ticket item, he tells the seller that he won't ship until the payment CLEARS paypal (and he moves the $ to our bank account). It protects the seller...but yes, it can make a buyer nervous.
I don't know how this is different if a person uses a credit card through paypal. In the above case, it was a cash transaction.