It is Buddha telling the traders that they shouldn't trust whatever is told to them even if it comes from the so-called experts/gurus, family/friends, teachers, traditions etc. He told them that they should try to reason out the things being told to them, give these a try in their lives and only keep them if these suggestions/advice works for them. A more simplified version can be found here too: https://www.ling.upenn.edu/~beatrice/personal/buddhist-pract...

Hope it helps. :)