In my most recent game I've been developing a trader/businesswoman character who's neutral with everyone (except bandits, heh) to maximize profits from enterprises (i.e. no risk of sequestration due to war, minimal negative relationships with town lords). I was interested in seeing which businesses would run well without me having to supply them with goods or ferry their products elsewhere to sell.
First of all, a good rule of thumb is to try and focus on products that demand high prices in towns where the raw material needed to make them is relatively inexpensive. A good example is that in Veluca, olives are usually not too expensive (~75-100 in the market), but oil is always pricey (it is everywhere except occasionally in Suno). This is probably why oil presses in Veluca work out well for a lot of people. Another example is Ahmerrad where they have cheap iron but expensive tools, which makes building an ironworks profitable usually.
Other businesses that have done decently for me:
~ Tihr: wool weavery
~ Shariz: linen weavery (flax is cheap there for some reason, head 1 town over to Durquba and it's a different story)
~ Yalen: brewery (LOTS of cheap grain in the area; not a ton of income but it was cheap to build and I haven't seen a loss thus far)
~ Sargoth: tannery
~ Bariyye: linen weavery
A good thing to do is to ask the guild master how trade is in his town; he will tell you what goods they are in need of. If you see a product that is in demand, and the GM doesn't also mention that they need the raw mats needed to make that product, then that might be the product you'd want to invest in. So, if he says they need wool cloth but doesn't also mention wool, then a wool weavery might be a profitable thing to build.
If you just can't figure out the supply/demand structure of a given town, or if you are rolling in dough, then you can't go wrong with a velvet weaver/dyeworks. They will turn a nice profit in every town except for Jelkala.