I do not understand, why not legitimate? and what is the main problem?
The sale was legitimate. The buyer was claiming it was illegitimate.
The main problem is that OP sold accepted Paypal payment for his Bitcoins. Anyone who buys anything through paypal can try to get a reversal by claiming that they, as the account owner, did not initiate the transaction. That means they get their money back
and your coins, and since Bitcoin transactions are irreversible, there's nothing you can do about it. It's standard practice to never accept Paypal payments for cryptos.
To OP, you can try to fight for your case by showing as much proof as you can to Paypal, but you're screwed for the most part.