In table: npc_types, run:
Code:
SELECT * FROM npc_types WHERE merchant_id > 0
That should show all NPCs that are set up as merchants in the game (my PEQ database shows 1753, which seems like a lot).
Table merchantlists is nothing but a list of what items the merchant has in what slot by merchant_id (set in table npc_types).
The item "price" seems to be set in table items, price field. I haven't verified this in code, but this is my understanding of merchants.