I've been working on some mods, and I finally got to making a merchant NPC. Unfortunately, I can't figure out how to let them sell things. I checked existing merchants, but I can't whatever setting controls it.

See these tutorials

http://cs.elderscrolls.com/constwiki/index.php/My_First_Shop

and

http://cs.elderscrolls.com/constwiki/index.php/Adding_Sellable_Items_To_Merchant_NPC

very important thing i found out.

Your NPC must have a set level, not scaling with the player. Else none of the merchant options will be avialable (all greyed out).