Hi!

Does mercantile skill have any automatic effect on buying/selling price if you don't haggle yourself? Does the price differ when selling something if you have a merc skill of 5 as opposite to 100?
I know disposition and fatigue automatically change the initial buying/selling price, but does mercantile only work when you haggle?

Yes it does have an automatic affect.
Mercantile affects the initial price of anything, so the better your Mercantile, the better the deal when you go visit a merchant.

It also helps you get better deals when you haggle. Meaning, you can make more changes to the price before they reject your offer.

The best way to get good deals is to make sure the merchant likes you. Get their Disposition over 80.

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There is a bug in the vanilla game, that causes merchants to give worse deals if your Mercantile skill is over 70.
The http://morrowind.nexusmods.com/mods/19510/ has a fix for this problem.

Thank you Pluto! That answered my question perfectly. I don't like haggling too much, but I think it's worth the effort especially combined with speechcraft (I'm using Economy Adjuster mod).

Related question. Do you get more Mercantile skill increase if you haggle when bartering?

You only get mercantile skill increases if you haggle iirc.

I know for sure that Mercantile can be raised by buying/selling (without haggling) But I;m not sure if you get more experience if you haggle.

Hrnchamd would probably know for sure, since he knows the code for most skills. Might be worth asking in the Code Patch thread (actually, I'm going to do that)
Because on a related note....
Bribes are supposed to increase your Mercantile skill, but it doesn't. Bribes increase your Speechcraft skill instead. However, bribery is still based off your Mercantile skill.
Going to see if he can balance that out so it makes more sense.

Ah, thanks for the correction. Will be interesting to know.

That's the way I've always understood it: more XP for better haggling. When I used to play characters I didn't want to level Merchantile on, I used to leave the slider alone because, it was said, you would get less experience that way. But I can't find anything to confirm this at UESP.