Under buyer protection, what does this mean

for items purchased off eBay, you must meet all of these requirements:
The seller’s PayPal checkout flow

And how does it affect buying something 2nd hand off the classifieds

Not sure if its the same thing but I sold something (not on ebay) and sent a money request to the buyer for payment.
They paid but then the money was held as pending and I couldn’t do anything from my end. Apparently the buyer had to confirm they had received the goods before the payment would go through. Stupid really as the payment sent including postage costs that couldn’t be accessed.