There are a lot of parts involved, but this is perfectly reasonable first as a first attempt at modding if you are prepared to learn.
The basic steps are:
Create the merchant NPC (UTC file)
Create the store inventory (UTM file)
Add the merchant and store to the the game
Create a dialogue (DLG file) so you can talk to the merchant and attach this to the Conversation field on the merchant
Write a script to open the store and attach this to a node in the dialogue file
Here are the essential tools required for the job:
KOTOR Tool to extract files
K-GFF to edit the merchant's UTC file, store inventory UTM file, and UTI files for any new items you might want to create
DLGEditor to edit the merchant's dialogue
NWNNSSComp to compile scripts
TSLPatcher and ERFEdit for compatibility if you intend to release the mod
You can find a rundown of the most common modding tools here.
There is also an archived tutorial from LucasForums on how to create a merchant here.
A couple more notes:
For my Jedi Tailor mod, I added a trigger (UTT file) to the dynamic area information (GIT file) that spawned the merchant and all other related objects. I felt that this had the smallest footprint and would therefore be best for compatibility.
I also added a few invisible placeable objects to keep track of local variables relating to the tailor's dialogue. KOTOR 1 typically uses global variables for this, and I feel this should be avoided unless strictly necessary, again for compatibility. KOTOR 2 has more local variables available, so the merchant could handle them all alone.