MerchantTradeItem has a bug with it's page system for instanced objects...
It never adds page index to current slot index, so instanced MerchantTradeItem list (with no database ID) can only have one page !
I modified most item storage procedures to take page as a multiplier for item index (page*MAX_ITEM_BY_PAGE+Slot_position)
Honestly I was unsure if this code was incomplete, but after editing it looks like someone could have left a "TODO" tag