Stores and merchants

From the official documentation:

"Stores are only accessed through script in a conversation – they are not objects that the player can detect. Stores have blueprints just like other types of objects. After you create your store’s blueprint, place it on the map normally. It is generally best to place your store next to the NPC. Check out the Sample Module for an example of a store."

Stores are usually started from a script attached to a conversation using the "ga_open_store" script. Place a store in the area and use the tag for the store as the first parameter.

Item costs 

The nMarkUp and nMarkDown parameters are for when you want to adjust a store's buy and sell price due to a plot driven situation or some other condition. Generally, you can leave these at 0 and 0.

You can create your own stores based on the pre-generated ones by right clicking and choosing copy blueprint. Note that when you do this, the seling and buying percentages are both 0. If you don't fix this, everything in your store will have a base cost of 1.

Thanks to seryn for doing the leg work to figure out that the OC uses a standard sell price of 100% when players purchase items and a 40% of sell price when the store buys from the player. These options are located in the properties window of the store. This is also the balance that Berliad's FRW character creator uses.

Plugins

If you're having problems working with items in stores, you may want to consider Lazjen's CPS Inventory Manager. Some features in this plugin include:

Issues with stores in patch 1.05

GodBeastX, Ash Darkwood and others recently reported a problem with patch 1.05 where you can no longer  set the buy and sell list of item types for stores. Apparently this worked fine in 1.05 beta, but now throws an exception and makes it impossible to set up these lists in 1.05 final.

The bug was reported and acknowledged by Rob McGinnis, Obsidian Community Coordinator.

Check out the thread here:
http://nwn2forums.bioware.com/forums/viewtopic.html?topic=560804&forum=113&sp=0