Once a BOA is bought it registers the aquisition with not only the realm database or record but also the account. When creating a new character on a diffrent server to the one with BOA items bought, a vendor at every start area will have the full selection of BOA items, this vendor can also exist in major cities. The only BOA items available (not blanked out and red) are those that match IDs in the player's BattleNet account. Some players will have a multiple of a boa item so it can simply dispense that number to the player, at any time.
When you create your new character you can access this vendor and choose to take the BOA items you have on your account, once done they are blanked and red, this means they are dynamic in that if you buy another duplicate, upon accessing that vendor it will be available.
The player may choose not to take all available in the thier inventory and can later access the ones in the major city to retrieve them.
Accessing the vendor on another new character on that server will result in simply blanked out items which you already have in your inventory on that server, however items you have not yet retrieved will still be there and available.
EX: player creates a new character on a new server, they have a dagger boa 2 chest pieces and two trinkets, the first character takes the dagger and a chest piece and 2 trinkets.
The player later creates another character, the dagger is no longer selectable nor the trinkets or the previously taken chest piece but the other chest piece is.
The player creates a third character on the same realm, when accessing the vendor, none of the items can be taken.
The realm DB is the dynamic update of character inventory, play ect.. but the BOA items in a similar vein to the purchased mounts and pets are created via a static record from the BattleNet account itself. The items cannot be duplicated on a single realm due to a record of how many on that realm. If the player decides to delete all characters on the realm then no inventory exists to duplicate. Transfers to another server with duplicate BOA items may cause an issue but the transfer can check this as the BattelNet record only increments duplicate BOA items upon purchase, upon transfer if it finds the inventory now exceeds that number then the BOA on the transfer Id is removed.
This also allows for scaling in that the guild BattleNet items can be included if desired.
This also means that as soon as a new BOA item is purchased on any realm it is added to the BattleNET account record, once you login to another realm it can check this and if you visit the vendor you will see that the BOA item is now available to you.
EX: player purchased a dagger BOA on realmA then logs in to realmB, player logs onto a character (level is irrelevant) and goes to the vendor and will notice that the dagger is now available to them, if not taken at that time it will stay available to any new or exiting character on that realm till put into the players inventory.
This simply has the realmDB handling the duplicates and availability with the BattleNet DB being the initialiser. BattleNet DB init are used already with regards to purchased items but relate to every new character instead of a realm.