I just messaged Leandra about this~ For the moment, that feature isn't automated. You have to change that fashion's name to say "Reserved for ___" and put it up for sale for whatever amount they offered.
Yes, I'm working on the coding to make it easier. For now the seller gets a message from the person wanting to make an offer and a link to the fashion below it. The price they offered is the message title and they can reply to contact the buyer about it. Going to work on this soon as I get a chance to code :)