If they are listed on eBay then they can simply buy them.

I would be very wary. As said you will have no seller protection. Also, this is the type of thing scammers do. They send you an email stating the money is in your Paypal account (that looks like a Paypal email) but will be held in escrow until the buyer confirms delivery. There is no money - Paypal do not do an escrow service in this country, if at all. You will end up with no money and no items. Or the email they send will have a link to Paypal which will lead you to their phishing page, where you will give them your password.

Tell them to buy through your listings. If you don't have all of them listed, list the rest and give the buyer the item number/s.

There are really no valid reasons for a buyer to WANT to initiate an off-eBay transaction.

Sending an invoice is easy, but I will decline to tell you how. You will only either get scammed, or not make the sale anyway.