The problem is chargebacks, paypal, credit cards etc. can be charged back by the customer, that is after they've made a purchase using paypal or their credit card they can just call up their company and say it wasn't them or even they changed their mind.
The result is that the paypal/creditcard customer get their money back, the merchant loses the money AND the goods, and probably also face a fine of some kind.
Once bitcoins are sent, they're gone, no charge back (merchants love this), accepting payment for bitcoins with paypal or creditcards is likely going to result in the vendor losing their bitcoin and their other money.
What happens is scammers use stolen creditcards or paypal accounts to buy the coins (which are then theirs), and the real owner of the card or paypal account cancels the payment.
Never accept paypal or creditcard for bitcoin.
If you want to buy bitcoins, use a bitcoin exchange. Right now the easiest and cheapest to use is Intersango, they have Euro, GBP, and USD bank accounts that you can transfer directly into relatively cheap and quickly.
I'm the community manager for Intersango, I am Nefario!
This guy talking about scams, trust him! He knows what he is talking about! Intersango is the single biggest scammer exchange out there!