Shopkeeps, Haggling, and the Art of Trade

How to dictate the terms of ingame bartering, trade, and haggling? How to deal with ingame arguments over splitting loot? Well, there are a few ways to do things, and a few ways to prepare.

It's good to discuss the ingame system for gathering loot beforehand. Many players like individual assets, by splitting loot into equal shares. Others prefer a Party Bank system, where all gathered loot goes into a communal treasury, and when needed, they can withdraw shares of it for things like personal spending, or purchases. Another method(rarely used, only ever played a game with this system once) is the award method, where players ask for loot from the DM, who awards it according to a Need/Want system. Those who need roll against each other for the item, in the case of want, it is determined by interested party members rolling against each other.

If there is loot that is found, in the case of the first two, which is, let's say, a +1 Longsword with a fire effect, then it becomes part of the accrued wealth of their current endeavor. A player may want it, but their total share of the gathered wealth is short by 200 gold. They can remove a share of 200 gold from the party bank, or in the other system, pay 200 gold into the accrued wealth of the party, to make it even for the others by taking the item. Now let's say that two players want it, a Paladin of Pelor, and an Elven Fighter. They can't decide who should have it, and they both want it. They could resolve this by entering into a bidding war, putting their wealth against the other for the item, paying out the share for the party, plus whatever further they're willing to pay the other party who wants it. Or they may decide to settle it via die roll; Highest roll takes it, after paying the share.

For intown RP, you are, to all intents and purposes as the DM, the NPCs voices, characteristics, and nature. A surly Dwarf selling armors, a sly gnome offering magical trinkets, a flighty airheaded human selling tools, a deep voiced half-orc offering the finest vintages of the Elderwood, a slightly deranged elven alchemist babbling about requiring more Toadmoss...
However, this is not as challenging as it sounds. You just need a bit of time for preparation. Write down a list of names, races, kinds of stores, and two outstanding features(Accent and a facial feature are easiest: Stereotypical fantasy accents, facial hair, scars, an eyepatch, look like they're sucking a lemon, always smiling, slicked back hair, etc.) If your players will be spending time in the same town, they can visit the same merchants again and again.
As for bartering, they can make checks against each other, such as Diplomacy, and have the shopkeepers check against the player with their Sense Motive(for ease, roll a d4 for their modifier for sense motive, or a d10 for an experienced shop-keep). Haggling checks could be broken down into a d20 system for ease of use; 1-7 means the haggle fails, full price. 8-14, the price is reduced by 5%. 15-19, price is reduced by 10%. Natural 20, price is reduced by 10%, and comes with a similar item of 10% or less of total value of item being haggled for.

As DM, your word is final on such things, though. If your players try to intimidate a shopkeeper for better prices, the players may very well encounter difficulties down the road. The shopkeeper may report them to the town guard; He may let the other merchants of the town know, leading to higher initial prices for goods. If they bluff that the items are worth more than they appear, and it falls through, the shopkeeper will press charges for fraud in a civil court, or take the matter to the local Lord or Noble. If the players haggle too much, the shopkeeper may declare that there will be no haggling, or even throw the players out.