First you follow the seller's instructions...such as post "I will take or buy the knife", or you send him an email with your intent to purchase.
You generally use your paypal account to send payment.
Seller will post or send you his paypal account address to send money to.
Sometimes they ask for P.O. Money order.
Seller will then send you his mailing address.
As only a registered member, you do not have PM, private messaging, privileges.
This is only a quickie answer, just read and remember what you read. Also read the instructions at the top of each section.