Make sire the items you are trading for this purpose arent already in the market, or the caravan. A common thing people like to do is use mats from blacksmithing, sell all of it but steel to the caravan, the steel to city. Buy it all back from both, then just sell again and this is where you start yo get points.
While following rumours from caravans and clever investments in certain types of items for trade is certainly the best way to make lots of money, there are a few more ways that you can make a quick buck. Here are just a few of them: Completing quests can give you a small cash injection. Go to a settlement and click on a person of influence with One way of doing influence gains is raiding enemy villages and such, if you wish to fight militias and such. If you raid the village it will burn them up, so many force them to give up loot works better. Your relations with people takes a nosedive though since you are targeting commoners. #1.