I have a simple question, I made a house mod where you have to buy the key to the house from a local merchant, but I can't figure out how to give the merchant the key. Also how can you make it so they buy additional equipment like ingredients and potions? Thanks.

To give the key to the merchant, find his ID under the NPC list in the object window. Either double-click or right-click and select "Edit", and the NPC window for the merchant should pop up. From there you just find the key in the object window and drag it into the NPC's inventory.

If you select the "AI" button in the merchant's NPC window, and go to the Services tab (I believe), there should be a list of boxes you can check that will allow the merchant to trade that selected item type. If they're all grayed out, go back to the NPC window and un-check "Auto-Calc".