I got trading to 275 within a few years. My most important discovery was that an average price is not an average price. You can get for example a lot of grain in Askar and Omor for 6 denars. Cities in trouble will buy that for 18 but the money they pay quickly goes down. @ a price of 12 grain no longer sells "red" but reaches average price. But you don't have to stop selling then. You can sell it down to 10 a piece or so and still make a profit. There is a sturgian town in the northwest of the map where you can bulk buy fish for 6 denars too, fish makes even greater profits than grain and you can sell hundreds of units of fish above average price in landlocked towns. Those give the big turnarounds given you have quite a lot of mules, at least a hundred or more as for trading big in fish/grain you need a lot of capacity.
Another thing is villages on the way also sell stuff, but often only at average price or even above. So fish in a fish village often costs 12 even though they got a hundred units or more. But you may be able to sell that fish for 20 in a town nearby (not the town where the village belongs to) so still profitable.
Trade rumours also help though until you get to that city things have often changed.
Along the way slaying some bandits and selling their stuff also helps with money, though selling loot will NOT ugrade your trade skill. Same for smithing parts, only iron ore and hardwood work towards trade skill.
So the big trade route is for me: Getting wool/meat/hides/fur around khergit territory, moving south, buying grain (@6-10)/beer(below 40)/dates (below 30) in aserai cities. (don't buy too much beer, I got a beer inflation in my game where beer was cheap everywhere thanks to too much breweries or so). Then move to vlandia, sell grain/dates there and buy olives below 30 or so and oil below 100. Then battania, get hardwood there in hardwood villages and buy wine and grapes if you get some cheap. Also iron ore if a decent price is there and take the green/cheap stuff. Also hides and flax can get good profits, but you can't trade them in massive numbers like fish. Sell a few hides and price reaches average quickly.
Then move to sturgia (buy fish/fur/grain and cheap stuff) and from there into the empires where you just buy cheap/sell expensive. But you need to get a feeling for prices. Here are my price "feelings" (maybe not 100% accurate is from the head)
grain: buy below ten/ sell above 12
fish: buy below 12, sell above 15 or so.
olives: buy below 30, sell above 40
beer: buy below 40, sell above 50
dates: Buy below 25 or so and sell above 40
olives: about same as dates can't remember.
salt and flax are "good" trade goods, you can trade them in bigger numbers with profits. Buy salt below 20 and flax below 12, sell@ sometimes massive profits of salt above 50 and flax above 20.
Wool: Buy below 100, sell above.
forgot clay, also a good one, forgot the numbers but can give good profts^^
meat, cheese and butter are difficult trade goods. Rarely you will get some cheap. Hard to trade and little numbers of them.
Then the rare stuff, hides silver ore, iron ore, hardwood pottery etc. trade in small numbers or don't bother.
Leather: Leather is always scarce in my game. Can't trade it.
jewelry: buy below 200, sell above or don't bother, tiny numbers.
So if you want to progress quickly stick with the bulk stuff. All refined goods like oil and wine are very rare, especially wine. You sometimes get oil, and beer is the most abundant advanced good. Sometimes too abundant. So you can actually ignore the rare stuff and stick with grain/fish/dates/olives/grapes/flax/clay/salt and beer. The rest is often not worth the effort or only good for a little side denar here and there.
PS: hardwood is scarce everywhere and cheap nowhere. You can often only get it from villages and @average price if you are lucky. Buy below 40 or even 50 and sell above. the green/red price system doesn't work for hardwood, you can buy it red and still make profits. Better still use it for smithing. The crafted weapons you can make can quickly reach massive values. I can make a javelin with a hardwood shaft, iron tip and wrought iron accessoire and sell it for 90 k now^^ No need to make expensive steel/fine steel/thamascene steel. To level smithing quickly, all you need is the smithing skill that gives you crude iron everytime you smelt, which basically means you get endless iron. (if you also smelt your crafted weapons while leveling smithing). All you need is charcoal and that means a lot of hardwood, I mean thousands of it. Tip: "smelt" pitchforks for 3 hardwood^^
Also there is a hardwood axis. From battania to khuzait, in the north I wrote myself down the hardwood route from village to village. From east to west there are 15 hardwood villages. I start in: odrysa, soutern empire, then omrotok, khuzait, then tepes, khuzait, then temhern, northern empire, then alebat, sturgia, then west of it, agalmon, sturgie, then west of that uthelaim, battania, then west of that andurn, battania, and west of that ebereth, battania, and then west from there near each other Tor Leiad and Glenlithing, battania. West of that the last villlage is the only vlandian hardwood village, Mareiren.
From there go east again and visit the last three: montos western empire, then popsia southern empire, then hetania northern empire.
That is the big route. Can be quicker though to stick around the battanian villages and buy all their hardwood. As inventories of villages resupply qickly.
PPS: Ah yes buy workshops, tanneries make you 400 profit almost everywhere
in khuzait territory, tanneries and wool workshops are fine. They don't have grain but a lot of sheep villages. Check for wanderers with N (encyclopedia) and get those with medicine/scouting and high attributes regarding charm/intelligence. They make great leaders of caravans for money (caravans don't up your trade skill yet it is a bug). Or make spin off parties with them, give them some recruits and in time they will level nicely. Engineers you will need later as governors but you need to reach clan tier 4 or so first and have fiefs etc. So don't have your companions in party as surgeon etc, do all yourself to level your character quicker. It's painful at the start but it well worth it, when you level scouting and medicine etc yourself. engineering levels ultra slow and you need to do a lot of sieges for that, so an engineer with 80/100 skill can be very good... same for leadership, you will need to lead armies a lot of times to level that. Painfully slow.