this is great, but more features need to be added, for example: need to add the ability to remove your item from the auction (they will be stored in his "private items"), add the /auc command, add privileged permission to list more items, for example: - reyd.auction .amount.10 (this player can create 10 lots)...
and also, so that there are no oddities with the fact that the item remains at the auction even after the purchase, you can make a new window (in the chest) to ask the player: "Are you sure you want to purchase this item ", also if some player has already purchased an item and another has already purchased it, you can make a message: "Someone has already bought this item", you can also make a button to refresh the page (in order to check the availability of an item at a given second, do not did someone buy it?