Paypal get charged by the credit card and debit companies for processing payments. They can't do it for free.
They don't charge if you are the buyer.
When you think of all the 'free' protection they offer buyers and sellers it's an excellent service.
Much more convenient than accepting cheques, and often cheaper too when you take in to account the time taken to get to the bank.
If you look at the PayPal fees, they generally charge 3.4% per transaction plus 20p. If the item was sold outside the UK, then they add another 0.5% to handle the currency exchange.
Wait til you sell something get paid with paypal and then they hang onto your money for 21 days because they say it's a ' high risk' item. despite having (grudgingly) used them for ages i got hit like that when I sold my old i-phone.