Ultima VI: The False Prophet/Trading merchants

Cereal trade
Buy sacks of grains from Linda in Britain for 2 Gold each; travel south to Paws and sell them to Grison for 3 Gold each.

Grison mills the grains into flour. Buy sacks of flour from him for 4 Gold each. Travel back to Britain and sell the flour to Cullen for 5 Gold per sack.

In New Magincia
Aurendir sells bales of wool for 5 Gold each. Charlotte buys bales of wool for 10 Gold each. The net profit is 5 Gold per bale, or +100%.

Charlotte also sells cloth for 25 Gold, but it can be sold (in Paws) for 20 Gold only. This would be a bad deal, so avoid it.

In Paws
You can buy bales of wool from Dorin for 5 Gold each and sell it to Arbeth for 6 Gold each. The net profit is 1 Gold per bale, or +20%: this trade it is way more convenient in New Magincia (see above).

Then, you can buy spools of thread from Arbeth for 3 Gold each and sell to Thindle for 6 Gold each. The net profit is 3 Gold per spool, or +100%.

Thindle sells cloth for 15 Gold apiece. You can sell it to Marissa for 20 Gold. The net profit is 5 Gold, or +33%.

Marissa sells tunics and dresses, but these items cannot be sold, nor they have any known use in the game.

Export from Paws
Fyodor in Britain buys bales of wool for 8 Gold each. It is more convenient than selling it to Arbeth in Paws, but still less profitable than to Charlotte in New Magincia. Also, each bale weighs 16 stones (about 100 kg), so the Companions can carry very few. Fyodor also sells thread for 5 Gold and cloth for 20 Gold: these are more expensive than in Paws, thus you will get lower profits. All in all, Fyodor is the first character in the game who introduces you to the textile business, but he is out from the trade.

Leonard in Yew buys spools of thread for 8 Gold each. His price is the best in Britannia! If you buy thread in Paws and bring it here, the net profit is 5 Gold apiece, or +166%. The spools are light, too, and your party can carry a lot of them.

Glass trade
Dale the glassmaker in Minoc sells empty honey jars for 5 Gold each. Zeke the beekeper at Empath Abbey buys the jars for 7 Gold each.

If you own a ship, you can sail between Minoc and Empath Abbey and carry out this business. The trade will result in a net profit of 2 Gold per jar.

A trading route
The most profitable trade in the game is to carry spools of thread from Paws to Yew. The profit is 12.50 G/S (Gold per stone). The second best trade is to carry empty honey jars from Minoc to Empath Abbey.

Let's see how to to combine these two trades and get top profit with minimal travels.

Southwards trip
In Minoc, buy a ship from Peer. Then, buy as many glass jars as you can carry from Dale. Sail west and land at the bridge between the Shrine of Justice and Yew. Walk west to Empath Abbey and sell all the glass jars to Zeke. You will earn 2 Gold from each jar, and the total sale will easily profit hundreds of gold coins.

Leave the Abbey and travel south. Fight the enemies and loot their weapons. When you get to Britain, sell all the weapons to Max. Still in Britain, you can buy either spools of thread from Fyodor or sacks of grain from Linda. Thread is lighter than grain, and you can carry more.

Therefore, buy as many spools of thread as you can from Fyodor in Britain, and travel south to Paws. There, you can sell them to Thindle. You earn just 1 Gold per spool, for a total of about one hundred golds, but it is still the best trade from Britain to Paws.

Northwards trip
In Paws, buy as many spools of thread as you can from Arbeth. Travel (maybe on horseback) all the way north to Yew. Sell all the thread to Leonard. With a party of 8, you can easily earn more than 1000 Gold.

The trip from Paws to Yew is very long. You should better buy horses.

Continue east to your ship and sail to Minoc, where you can restart the cycle.

Horseshoes
Rufus in Britain and Harold in Trinsic sell horseshoes. A set of four costs 2 Gold.

Who buys horseshoes?