If I sell some btc with paypal. The transaction occurs and I get the paypal in my account. Then I use the paypal instantly. Will the buyer be able to reverse the transaction once the paypal has been used form my account?
Of course he can't retrieve if the PayPal funds is already been transfer out to your account but the buyer can report your account in case he want to dispute your transaction which might cause to closure of your PayPal in case you don't have proof that you have a legit transaction for that money.
It's very hard now to dispute on PayPal but still possible if the buyer has an evidence. This is a very transaction and I don't recommend it.