Hi guy, im about to create my first shop, (maybe not the building, but just an NPC)

Here my shop idea:

Exotic item shop/vendor

-Sell retextured item(i will download mod, take the texture file, copy the existing item, add the copied mesh, need no permission from the author, since personnal use
-Sell some unanchanted cool weapon
-Sell some styled robe/armor/weapon(like mythic dawn armor)
-Sell weapon replica
-Sell everything i want

Someone know a good location, that UOP dont modify, so my pluggin dont overwrite the fixe that UOP did to the cell, to put my vendor?
A suitable place for a exotic seller...

To find out what cells are modified by the UOP, open it in TES4Gecko or TES4Edit. Or, if you aren't familiar with those yet, open it in the CS and look for a cell in the cell list that doesn't have an asterisk (*) after its name (which indicates it is unmodified by the active plugin).

You should not be afraid to place your vendor in a cell modified by the UOP, however. Just pick the cell you want. If you notice anything amiss later, you can just copy the corrections the UOP made to the cell to your mod.

Adding a NPC to a cell will not cause any problems with other mods that change the cell (unless they completely alter the cell beyond recognition so the NPC ends up floating in mid air for example - something the UOP certainly doesn't do). Reason is that multiple mods can add stuff to the same cell without conflicting. Unlike leveled lists for example, which can only be altered by one mod unless you use Wrye Bash.

So basically you can add your NPC to any cell without worrying about conflicts.

So if i make a mod that add content XYZ cell, then download another mod that add XYZ content to XYZ cell then i enable unofficial patch, the content from both mod and the fixe should be in the cell??

And like i said, only add, not remove or change, i hate modifying what is already here.

Yes, multiple mods can add stuff to the same cell without conflicting. Only cell properties (light level, water height etc) can't be edited by two mods. Same goes for landscape and path grid records. But adding objects, actors, items, lights, whatever you can find in the object list - no problem.