PayPal have the following (I may not be completely covering all of the polices, here are those that I remember),
----Buyers----
- Purchase Protection (Virtual / Order of services)
- Buyer Protection (Physical with delivery)
- Refunded Returns (Delivery / Shipping fee)
----Sellers----
- Sellers' Protection (This covers many things, will state the general majority below)
----PayPal----
- Unauthorized Transactions (These will always result in a charge back notice from PayPal if you happen to come across it)
Now this is how PayPal works, as a Seller. For physical sales, after an order comes through and you provide shipping tracking information and mark payment complete, the buyer will not be able to dispute based on the grounds of "I didn't receive the item". They can however try to claim damaged /wrong goods delivered etc, it's good practice to keep photo records of shipments or invoices if you are drop-shipping them. This will help your dispute, and usually you win. If you don't, call PayPal to resolve it. They like the sound of customer voices, just try not to yell at them.
For ticket sales (if you happened to be selling tickets...), .... I don't know I don't sell them.
For online services, this will be tricky and I personally believe they have an semi-automated dispute resolution system that make decisions based heavily on account reputation and track records. Proper invoicing notifications, clear description of product and display of price on your sales site will help in this case.
For unauthorized transactions, prolong you fulfilled your sellers' responsibilities unless with the exception of frequent unauthorized transactions occurrence on your account, PayPal will leave you alone after you provided the sufficient evidence of delivery records.
For the recent years after PayPal's policy revamp, sellers are pretty much protected. The only thing that never changes is that PayPal will block you trying to make a payment randomly out of their own free will, and the customer support will be unable to provide any reasons for you and just keep asking you to pay again.
P.S: Above applies only to personally registered and verified seller accounts, I have no idea what the procedure goes if you are buying PayPal accounts online to do eBay/ Shopify drop-shipping or w/e.