It's really simple
On Paypal's main page (the page you get when you log in), there is a sidebar on the left. On the 'Selling Tools' section, you can select 'Create and manage invoices'. Once on that page, you can click 'create invoice', and fill out the form from there
Alternatively, a buyer can send you money when given your Paypal email address.
I hope that helps somewhat
Also, when you get round to selling them, can you send me a list of your G4's?