Someone looking to sell bitcoins on localbitcoins.com will see prices less than major exchanges for in-person cash purchases. I guess that makes sense, because the seller is willing to accept less cash when he gets it instantly, in person.
However, the prices for for online trades are quite a bit higher than major exchanges. The current Bitstamp rate is US$250/BTC, while one trader on localbitcoins.com is offering US$339/BTC (paid using PayPal). Why is this trader willing to pay so much more than he would if he pays an exchange?
As for Paypal the prices are always higher while buying bitcoins with it , because of the risk of charge back. Along with that , most of the times you are also supposed to provide identification documents in order to provide proof.
As for other trading, most of the sellers just buy and sell on there and thats the mark up they charge. They also have to risk and wait the trading time and have to pay by hoping the price doesn't fluctuate much in that time. I am a seller on the site and have had to wait upto 2 days because once the transaction was reversed right after the trade.