I might not be the best to explain this so if anyone else is willing to respond, they should.
The buying benefits of paypal are great. It gets money quickly to the seller which saves time. Obviously your LEGO is shipped to you much sooner. Sending payment is free.
All of the problems I have heard is from sellers who accepted money from paypal. There are fees for recieving payments which vary depending on what type of account you have.
I assume that you will only be sending money. You need an email account, bank account, and credit/debit card. When you sign up you enter this information. Paypal sends two small deposits to your bank account. Once you have your bank statement you can see these amounts. (an example would be $0.17 and $0.38 ) By entering these numbers at paypal, you verify your account and can send money.
Most sent money is taken directly from your bank account or credit card to the seller. Many sellers like balance funded payment, not credit, because this reduces the seller fees.
It would be good to have a certain bank account for paypal. A separate account without very much money in it. If someone got ahold of your paypal info, they could drain your account or run up a credit bill. Watch out for scams. There are a lot of fake sites, looking just like paypal, that send out mass emails for people to update their info. Ignore these! Only enter info if the beginning of the webpage is https://www.paypal.com
That is very important.
You can also search on the web for sites telling about paypal horror stories - which do exist. Some people like to use paypal and others avoid it. Personally, I think that it is a good tool for buyers if you take proper precautions.