Wines that are sold 'in bond' have not had the duty and VAT paid on them.
They must be stored in an authorised bonded warehouse.
If and when you want to take them out of bond, you pay the duty and VAT on the original price paid for the wine. If you sell wines in bond to another customer who wants to store them in bond, they are transferred without any duty or VAT being paid.
If the wine is stored in bond, you are almost certainly guaranteed they are stored in the right conditions, which makes them more enjoyable for the drinker and more attractive to buyers if you are selling them on.