Is there a mod that can make Oblivion buying & selling to be like http://fallout.wikia.com/wiki/Barter#Fallout_3

In Fallout 3/New Vegas, the price of the items you are buying or selling are directly tied to your barter skill and your Charisma stat. There is no haggling and no need to guess the optimal buying and selling price for a merchant.

Is there a mod that can automatically set the prices of items being bought or sold to their optimal values (depending on the character and the merchant's Mercantile skill and disposition) and without the need to haggle?

I haven't played Fallout 3, but what you describe is just one of the many useful features in Enhanced Economy .

Thanks, I'll check that out.

From http://tesnexus.com/downloads/file.php?id=25078, it seems that this is the feature I want:

Yes, that is what you want. While you are there, you probably want almost all of the other features of that mod. The mod truly is golden. IT also incorporates other features from Fallout 3/NV, like when you buy from merchants, they actually lose money. Try the mod out and feel the giant difference it makes.

I was actually using http://www.tesnexus.com/downloads/file.php?id=4432 (without Living Economy - Items) before this, so some of Enhanced Economy's features are already known to me. I have now replaced Living Economy with Enhanced Economy in my Oblivion installation. The Fallout 3/NV-style barter is nice as I am also currently playing FA: New Vegas. Will there be bugs if I just simply uninstalled Living Economy and installed this? I haven't invested in any stores yet, and my save game was not while buying/selling (not a place close to a merchant).

As long as you make sure to not be inside a store in the last savegame before uninstalling LE, you should be perfectly fine.