This option is no longer possible, unfortunately (I believe the function was being abused in some way - I have vague memories of people mentioning it being used to manipulate FB in some way, anyway o_O).

The only thing you can do now is create a listing with minimal info in the title and preferably at an agreed-upon time with the buyer, to minimise the risk someone else will buy it. (I usually have Custom listing reserved for: [member ID] as the title, and send the buyer the item number as soon as the item is listed.

Another method (credited to k1ooo-slr-sales), is to have an unusually high Buy It Now price, then use the Best Offer function and let the buyer know they just need to offer the agreed amount. That way, it's highly unlikely anyone will BIN the item, and you can reject any offers that come through from other members.