Morrowind:Commerce

Commerce in Morrowind is fairly simple. Vendor prices are determined by both your Disposition with the merchant and your Mercantile skill. Disposition can be raised via persuasion as well as by Reputation, both general Reputation and Faction Reputation if the merchant is a member of a faction. Mercantile can be raised simply by buying and selling items, or bribing people to increase their Disposition. These factors will affect all types of commerce, including Merchants, Spell Merchants, Enchanters, Spellmakers, Repairs, Trainers, and Transport. Very few prices in Morrowind are non-negotiable. (Examples include donations at Shrines or paying for a bed at Taverns, though these are in general very negligible.)

Base Value
Base value is the value an item has displayed in your inventory.  You will almost never be able to buy or sell an item at base value, unless you exploit the Creature Merchants. All other vendors will raise their purchase price and lower their buying price depending on how much they like you. Additionally, if the item is a weapon or armor, the buying price will be lowered if it is damaged. You should always repair items prior to selling them to get the best price. (Best of course is to repair them yourself, rather than pay for the service.) Enchanted items with a charge on them will sell at full value even if their charge is completely depleted, so there's no need to recharge them first.

Merchants' Gold
All merchants have a supply of gold on hand with which to purchase items. If you try to sell a merchant something which is worth more money than he has on him, he will give you only the amount of gold he has, and you will not be able to sell him anything else for 24 hours. After this period, the merchant's gold is "re-filled" to the same amount. One way to get your money's worth for these more expensive items is to first purchase something expensive from the vendor, temporarily increasing his available gold, then selling your expensive item. You can then take the item(s) you purchased and sell them to other vendors, or wait a day and sell them back to the same vendor. Merchants will also keep items you have sold them in the past, so if you sell them some items for their maximum gold, you can use these items to purchase later in order to sell him more expensive items.

Another technique to increase a vendor's gold will work only with merchants who are also Enchanters or Spellmakers. Most custom-made enchantments and spells are very expensive, so immediately after purchasing one, you should then take the opportunity to sell your expensive items back to the merchant, thus recouping the cost of the services. The trick is finding merchants who will purchase the items you wish to sell and who also provide one of these services, of which there are few.

Yet a third (and highly unorthodox) method is to actually sell the merchant gold. All merchants will purchase gold, and every gold piece you sell them increases their supply of available gold. Best of all, when the merchant's gold stock resets, whatever gold you sold them will remain in their inventory. Thus, when you return the next day, you can now sell them whatever expensive items you want, and continue doing so every day from then on. (Note that after a week of not visiting a vendor, their inventory may reset to the default values.)

Creature Merchants
There are two special merchants who are favored because they are creatures, and thus have no Disposition. This means that they will always buy and sell items at their base value, regardless of your Mercantile skill, Personality, or Reputation. They also have the advantage of being richer than most of the NPC merchants. They are:
Creeper — A scamp merchant living with the Orcs in Ghorak Manor in Caldera. It will buy anything other than alchemy ingredients, non-magical clothes, and books. It has 5000 gold available for trade.
Mudcrab (merchant) — A Mudcrab merchant found on a small island just east of the Dwemer Ruin of Mzahnch. It is a little bit more selective(does not buy soul gems) than Creeper, but it will still buy weapons and armor, which tend to be the most expensive items, and it has 10000 gold available for trade, more than any other merchant in the base game. If you are looking for it, be aware that it looks exactly like any other Mudcrab, and is named "Mudcrab", as well. To determine the correct mudcrab, either wait to see if it attacks or simply try to talk to it.