If you are on PC i recommend using the AutoTreader mod (as long as you have the game on steam it's even in the workshop), it might not make Trading XP any faster but it saves a bunch of time as you just need a click to buy and sell.
Best setting i found for it from an user on Reddit were this:
Buy under 80
Sell above 105
Price search radius UNLIMITED.
total Inventroy space used 101
Max of inventory filled by 1 item 10%
Haven't had a good experience with the setting based on green and red (the mod seems to act funky if you don't use personal settings).
Most of the money i made was coming from Tools, Silk, Pottery and Fur.
Horses seem to be quite bad to trade for, but i think the extra speed and the carry capacity is worth it, while providing some nice profits when you can sell some.
Haven't tried grazing animals yet, but they should be quite good as well. Only issue is that you have to buy them instead of extra mules if you don't mind sacrificing some speed for extra cargo
As for towns you should always stop by Sanala (for huge profits mainly) , Myzea and Dunglanys for silk. Epicrotea and Rovalt for tools. And stop by the 2 villages above Varcheg for Fur, then stop by Varcheg itself. And don't forget to do the same with Ostican and the fur villages around it as well.
You can skip most of the Khuzait and Sturgia didn't find them to be great for deals.
I think i got around 1 point in trading every 2-3 towns past 250 and all i needed to do was stop, click a button and go to the next town
((