Most likely the price for tools in that town is low then add in taxes and relationship with town/ faction and trade skill.
i scout out towns by having a few choice items like salt, velvet, spice, tools on me (only 1 of each) and compare who likes what.
the reverse is true when buying a caravan.. find a town who will buy tools for 30 or less, sell them the one your carrying (if they have none in stock to enable a tools caravan), arrange a caravan, buy back your tool from the merchant then run the caravan.. oil / tools to Moscow and T'ver seems to be good.