Make sure you have the Spawnable column set to 1.You have to edit TreasureClassEx to get it included in drops, unless it is one of the 150 "virtual" drops (e.g. weap18, armo45). To be a virtual drop, it must be defined in weapons.txt or armor.txt.
To sell in stores, go to the defining txt file and scan right until you see a lot of columns with vendor names. For example, here is Charsi selling arrows in misc.txt:
CharsiMin 3
CharsiMax 5
CharsiMagicMin 0
CharsiMagicMax 0
CharsiMagicLvl 255
Edit min and max so you have a chance to see an item sold. If you want the item always sold, set column PermStoreItem to 1.