Well... if your play style is "pure trade, without building" I wouldn't advise playing any campaign/scenario missions, building is a must in most of them. If you, however, want to widen your horizons, it would be best to play sandbox mode and play a bit with buildings first before starting campaign/scenario. Believe me, buildings are a lot more complicated than it may seem at the beginning.

First of all you cannot star building to early. You will have to pay wages to your workers and they are quite big. Furthermore you have to pay them regularly and more often then you are able to trade. Don't get me wrong, it's still profitable but it may happen that you have spend ~10000 for wages first and then you will earn ~15000 (or even more!) out of it. But you need this 10000 in cash first so don't build anything before you acquire reasonable reserves.

Focus on high-end products first then build complementary facilities to aid them. Example: find a city where people are experts in creating iron goods. Build a proper factory there. You'll have to supply them with pig iron manually but you'll be able to make a decent profit nevertheless (just remember about my previous point). Then after some time, when you are rich enough, find a place with expert miners so that you can produce pig iron yourself. It won't be the same city so you'll have to arrange the transport but in this way the production of your iron goods will be dirty cheap! You'll never be able to buy them at that price anywhere! And then your profit from it will be pure awesome!

Some businesses are not really profitable but extremely useful in certain situations. For example you won't earn much with bricks, timber and hemp but you can use them to please a prince and reduce your expenses when you build something else (especially if that's a new city you were assigned to build!)

The side effect of building businesses is that they attract more people and the population of a city grows rapidly. You can imagine how much you can earn from rents in this situation (remember to build a few houses before you expand the city). But the biggest advantage is that the bigger a city is the higher are demands. So by building a lot of factories you can influence the market. You can get tens of thousands in a very short time just by feeding those crowds (of course we are talking about the food you produce yourself, perhaps in another city).

That's just a few examples, I may even say it's the tip of the iceberg. Building part is at least as complex as the trade itself in Patrician III and I can wholeheartedly encourage you to give it a try.