As the topic states. What is the best way to RP a traveling merchant, making a profit out of it, without too much adventuring? I am talking about buying things in one city, and selling them for a profit in another? are there any mods or anything for this?

There is no such thing as selling for a profit even with speech at 100 and with all perks.

Perhaps there might be a mod for that.

You could also look into smithing as well. Just create your own weapon/armor/jewelry and sell it that way.

Take up a trade, such as weapon/armour smith, alchemist, or enchanter, then make goodies, and wander around selling them. That's how I got humungulously wealthy with my current character.

Alternatively, if you like to use mods, install Amazing Follower Tweaks (AFT), hire yourself two or three mercenaries or take housecarls, and go adventuring but let them do all the fighting...you can be the fat merchant nabob directing the show and reaping the riches won you by your hired thugs.

Yes, there are.Thomas Kaira's http://skyrim.nexusmods.com/mods/11919 is the mod for you. Among many other changes, the mod introduces regional values. Some materials are more plentiful in some holds and sell for less there; the same material will be less plentiful in another hold and will be more valuable to merchants in that hold. Food merchants will pay more for food items, blacksmiths will pay less for food items. And so on and so forth. You can spend an entire game travelling between cities, buying cheap and selling dear.