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?
Yes if the claim is approved by the PayPal then your funds will be debited from PayPal whenever you credit funds to your account so you can take fund and leave your PayPal account forever or better avoid trading with untrusted traders when it comes to p2p trading involves paypal as payment mode.