This table holds the vendor data for all NPCs that sell items or currencies. A vendor can only hold up to 150 items (15 pages), it is hardcoded in the emulator and if you change it, the client will crash.
The price (in gold) for each item or currency is defined in its item template related entry as item_template.BuyPrice. Special costs (honor, tokens etc...) are defined in this table in the column ExtendedCost.
If you open the vendor's window in GM mode, you will see all the items sold by the vendor. If you disable GM mode, you will see the items sold like a normal player (ex: if you can't use an item and cannot trade it, you won't see it listed).
The ID of the creature. See creature_template.entry.
Position of the item when the vendor window opens. It starts from 0 to x, from top to bottom, and from left to right. Note: if you have a list of items at slot 0, and you edit one of them to be at slot 1 or any number, that item will always be at the end because all the other items are in slot 0.
The item ID. See item_template.entry.
The maximum number of items carried by the vendor at any time. If you want the vendor to carry an unlimited number of the item then set this to 0 else set it to any positive number. In the image below the maxcount value is circled.
Combined with maxcount, this field tells how often (in seconds) the vendor list is refreshed and the limited item copies are restocked. For limited item copies, every refresh, the quantity is increased by item_template.BuyCount .
The value here corresponds to the ID in ItemExtendedCost.dbc and that ID controls the item's non monetary price, be it honor points, arena points, different types of badges or any combination of the above.