Currently there are a total of 469 EVENT_ constants in the global table (each of them containing the events ID). Finding the right one can be tricky
Have you looked at those:
EVENT_BUY_RECEIPT = 131234
EVENT_SELL_RECEIPT = 131235
EVENT_BUYBACK_RECEIPT = 131236
EVENT_UPDATE_BUYBACK = 131233
Some other constant with sell or buy in thier name:
SI_ITEM_ACTION_SELL = 1829
SI_PROMPT_TITLE_SELL_ITEMS = 3546
SI_SELL_RECEIPT_FREE = 2035
SI_SELL_RECEIPT_MONEY = 2034
SLOT_TYPE_BUY_MULTIPLE = 8
SLOT_TYPE_STORE_BUY = 6
I also think that the "Buyback" area is considered another Character bag, similar to the account bank and the worn items:
BAG_BACKPACK = 1
BAG_BANK = 2
BAG_BUYBACK = 4
BAG_DELETE = 255
BAG_GUILDBANK = 3
BAG_TRANSFER = 5
BAG_WORN = 0