I noticed a distinct lack of a traders guide, so I decided to do a little research and write my own. This is meant as a general guide. Prices vary widely over time, but the areas that are predisposed to certain commodity prices seems to remain constant. All towns are listed left to right in order of best to worst price. Prices listed are with 10 trading, so assume a bit higher/lower if you have less trade. Also all cities are 0 relation, better relations will improve your margins.
The ordering of towns is from an average view of the 5 different save games over the course of several days each, and the top price will not always be at the listed town - it's just the town that averaged a higher price. Prices listed are in ranges, some quite broad. These are the general ranges of prices that I recorded in my testing of the towns listed.
Note that villages offer cheaper merchandise than the towns, sometimes less than 1/2 the price of towns. From my experience, the villages usually stock the same cheap good as the town they are near. For instance: Price of Oil in Suno is currently 208 in one game. The nearby village Nomar has several units of oil at a price of 111. All city's don't always have the good that they sell, either - in some cases I had to wait for several days before they got them in stock.
Nord Lands
Fish
Buy at: Wercheg, Tihr.
Sell at: Narra, Veluca, Halmar, Tulga
Buy/Sell prices: 5-30/55-70
Linen
Buy at: Sargoth
Sell at: Narra, Tulga, Halmar, Veluca,
Buy/Sell prices: 85-95/230-240
Vaegir Lands
Meat
Buy at: Rivacheg, Reyvadin
Sell at: Tihr, Halmar, Veluca, Narra
Buy/Sell prices: 15-40/75-90
Wool
Buy at: Reyvadin
Sell at: Narra, Yalen, Veluca, Tihr
Buy/Sell prices: 20-50/140-160
Iron
Buy at: Curaw
Sell at: Tihr, Yalen, Veluca, Narra
Buy/Sell prices: 60-90/280-320
Furs
Buy at: Khudan
Sell at: Yalen, Veluca, Narra, Tihr
Buy/Sell prices: 75-150+/435-475
Swadian Lands
Ale
Buy at: Praven
Sell at: Tihr, Rivacheg, Wercheg
Buy/Sell prices: 22-52/80-105
Wheat
Buy at: Uxkhal
Sell at: Tihr, Wercheg, Rivacheg
Buy/Sell prices: 12-40/80-100
Flour
Buy at: Uxkhal
Sell at: Tihr, Wercheg, Rivacheg
Buy/Sell prices: 26-50/95-110
Tools
Buy at: Dhirim
Sell at: Veluca, Narra, Tihr
Buy/Sell prices: 115-200/430-470
Oil
Buy at: Suno, Narra
Sell at: Wercheg, Curaw , Rivacheg, Khudan
Buy/Sell prices: 195-300/480-520
Rhodok Lands
Cheese
Buy at: Yalen, Jelkala
Sell at: Tulga, Rivacheg, Ichamur, Khudan, Wercheg
Buy/Sell prices: 35-60/100-110
Wine
Buy at: Veluca
Sell at: Rivacheg, Tihr, Wercheg, Khudan
Buy/Sell prices: 30-80/145-165
Velvet
Buy at: Jelkala
Sell at: Tihr, Rivacheg, Wercheg
Buy/Sell prices: 615-780/995-1025
Khergit Lands
Pottery
Buy at: Halmar
Sell at: Tihr, Yalen, Rivacheg, Wercheg
Buy/Sell prices: 15-30/130-150
Salt
Buy at: Ichamur
Sell at: Yalen, Suno, Rivacheg, Tihr
Buy/Sell prices: 65-90/270-280
Spice
Buy at: Tulga
Sell at: Yalen, Tihr, Wercheg
Buy/Sell prices: 200-470/915-1003
Also note that most goods are valuable at the most distant areas, with a few exceptions. One oddity was the price of oil - I found it for about 300 at Veluca and Narra as well as the lower prices in Suno, and in one game Suno was at 300 for oil too. The result is the distant southern lands pay very little for oil, so the best prices are northeast.
Since I only used 5 games with a few days in each as my sample, expect different results - but the general prices should be at least similar.