Just wondering if anyone could answer this....

So i sold a knife + all my stat trak csgo items to a guy for 550$ on paypal, and now he opened a case on paypal saying the payment was unauthorized and that he didn't receive his items when he did. i was wondering if anyone out there has been in this situation and could tell me how long it takes for paypal to review this case. also if someone could answer this. if you were in this situation before what did it take to win the case?

What he's doing is a chargeback, often times individuals do so to scam others. Sometimes these cases are legitimate and their accounts are indeed hijacked.

However, it sounds like someone is trying to scam you. It's difficult to win these cases as PayPal does not offer seller protection for intangible goods, like digital items. I definitely recommed filing a report against the player.

As for the case, gather as much as evidence as you can.

Yesterday I called paypal about the open claim and they gave me the funds after i showed them a screenshot and i told them he was scamming me. but if anyone wants to help me out some more then i put up a scam report on this guy just go here.
http://forums.steamrep.com/threads/76561198103076077-noob-paypal-scam.48203/