GoblinVendorFilter adds a small button to the merchant window. This button enables you filter the current merchant window items by itemType, subType, itemstats and equipLocation.

Filters will not appear on the dropdown if they either would filter-out nothing or everything (wouldn't make for a useful filter in that instance). The filter clears each time you close the merchant window the filter resets to ensure that you do not miss out on great deals next time you visit a merchant.