Is mercantile a thing? Can I buy resources like bricks and stuff and sell them in another town for profit?
No. Well sort of. There's quests for "find a gold nugget, sell it in this town, and we'll pay you" but they're measly. Like, buy a gem for 100, resell it for 30, get a quest turn in for 250 - that's like 200 or whatever profit, which is nice but not nearly enough to do much with. You're better off running kill quests as you get the quest rewards and the kill drops, and the exp. When you get to level 10 (or can get a single character to 10) you can build structures, then put the clay and wood and stuff to use building a market, which brings in gold daily!
If you are going to use the towns for selling loot anyway just buying all the gems you can afford when shopping and checking all the gem quests, then turning in all the gems, repeat. Can quickly get a lot of money+xp for going where you were going anyways.
Selling resources like that though is pretty useless. Girlinhat is right that it is better spent for buildings and stuff once you can afford another team worth of people to garrison. The only real exception is kingdom events. There was one point in Staberinde where the King ogranized all the active players into upgrading their and specifc towns, with kingdom sponsorships for their projects later. He also reduced taxes to a flat 1%.
Also note those resources are often better spent in towns to upgrade them as over the long term towns net you loads more dosh and can be made more secure but they have their own disadvantages. Elections, Approvals, guards, more garrisons, kings, other adventurers, etc. are all things you will have to deal with more than wilderness buildings. They aren't a constant stop for everybody through the wilderness and they can be set to just attack you. Cities are just more commonly looked at and I've walked in to a few and decided to sack them. I kill everything but the mayor and empty the chests. This is more to Npc towns but during kingdom wars... I can't promise anything.
I'm currently based in Amaranth (33,20) again. I'm 2-3lv and all my attack skill is low 20s and defence skill has a high of 30. I've got 7 people so far. Only 2 of them are at all squishy and I have 1000+g