It's a scam, for sure.
"The website cooperates with Paypal. Each customer can only purchase one product. If you buy more, please place another order. Thank you for your cooperation."
If you want to go through the hassle, just for kicks, use a credit card so you can do a chargeback in case PP decides to go against you despite proof that the items were never shipped to your place. They'll most likely ship whatever from somewhere near to another place near you, then upload that tracking number, and swear on their mother that they shipped it to you. At that point, you can either go through the hassle of disputing things and explaining yourself with PP, which will take a while, but usually resolve in your favor when you obtain the necessary proof (some form of communication from the shipping company saying that it was not delivered to your address - must include your address on that piece of paper/email). PP claims person will very likely ask you to go through the burden of obtaining that stuff in an official letter from the shipping co. I think it's excessive, usually, email suffices, especially given that they are supposed to have access to the proof that the seller uploaded which clearly should show that it was not delivered to your address.
At that point you can decide what you want to do: comply with PayPal and end the stress sooner, or take them to small claims court for your money, court fees, and damages. They'll probably settle. If you sue, your account may be terminated.
So, ask yourself really hard first: what would you like your next several weeks to be like.
