Well I think it's obvious - sell it as 'Pre-EU Pure British Heritage Soap' on eBay. Say it's a unique bar, none other known to exist, and then you can also sell the other 2/3 as unique  as well.....

Don't forget to remove the price label and replace it with one that says, say, £49.99 so that you can then ask for£99.99. If there are no takers just add #Brexit-approved to the advert and some 1di0t will then buy it.....