There is no way to do a real, private offer. Restricted to one buyer. That's why Padi says to list at a ridiculously high price, to deter anyone else buying it first.

If you list it like that make it clear to your buyer that you will accept the agreed price on it so they don't freak out 🙂

I would also put RESERVED in the listing title. So it's clear if it turns up in searches before the buyer sees it. Also a good idea to let the buyer know what time you will be listing it, so they can then put the item number in the search bar and find it before it goes "live" to everyone.