Step 1. Register with paypal (Bank details required by them)
Step 2. Sell/Buy item
Step 3. Obtain sellers paypal address (or send address to buyer if you are selling something)
Step 4. Log into paypal, enter address into recipient box (if buying) and send money over. If selling they will do this you just send them your paypal address.
Step 5. Send item or wait for item to arrive