Most of my biggest money has come from smelted metal sales to Orystia.
Although that's maybe because they're getting their shit pushed in by both Brittania and the North Empire. They're down to like 3 cities, so maybe they're really hurting for raw materials. They've consistently had high buy prices for metals for like the last 2 in game seasons.
But basically I do a run in to Brittania, buying up whatever is cheap, then go down south to Orystia/Sagot. By the time I reach Orystia I've got boatloads of weapons. I smelt them down, make between 5 and 6k, then head back north. I make more money if they're also in the market for Iron Ore since I can get that super cheap from Seonian as well. By the time I make the full circuit, a day's rest usually restores my party of 5's blacksmithing stamina so I can smelt down the 8 to 12 weapons I've collected. Doing this I've managed to push myself over 20k while comfortably supporting a warband of about 70 guys.
By and large it's my best money because there's almost no upfront investment. Compared to sinking 1-3k denars in goods then hoping I make that money back by the time I get wherever I'm going, it's just a few hundred in hardwood to smelt it all down. So even if the prices aren't great, it's still pure profit mostly and requires less running around checking prices and gambling that they'll stay steady with other goods. Of course, the metal market there could get oversaturated.....or collapse entirely if they get taken over. But it would seem to be that struggling nations make the best customers since their whole supply chain is on the rocks. And if the price isn't right....I just sit on it as I travel around. When Crude Iron is at 50+, Wrought is at 80+ and Iron is at 120+, I sell.
After another round or two of this, once I have enough of a cash base I'll start doing caravans.
Also what made a big difference for me is just always buying a couple horses during my stops when the price is low enough. Between that and a few camels I looted, I've got like 3500 carrying capacity and a movement speed of 5.7. It's weird to me still to have an inventory of 30+ horses and to keep buying more, but that's the name of the game in BL.