It doesn't matter if they're lame or spirited, saddle or hunter, all increase movement speed by the same amount. The more horses you carry the faster your world map speed is (up to a certain limit, naturally). Horses can't be traded, but are invaluable to a merchant. While regions' supply specialties will not change (Ichamur is always going to be the cheapest place for Salt), the demand characteristics are more fickle (Wercheg may start with the highest price for wine, but that it is unlikely to remain that way throughout the game).
Prices change continually, with and without your influence. Yellow labels beside the town names describe what they produce, the buy/sell prices, profit in gold, and profit in percent. Red arrows indicate the destination of each type of Goods.
