This. As an example, you can buy cinnabar for a measly 3 gil at the tradecraft vendor in any of the three city-states, as well as the vendor at the Alchemist guild. On the market however, NQ cinnabar sells for about 40 gil each, and while not selling quickly, does sell. People are lazy. Especially if they have a little excess gil from leveling another DoH or DoL. Rather than trying to find out where something is sold or obtained if they don't know, they'll just buy it off the MB without a second thought......
EDIT: I should say "on the market for my server" it sells for about 40 gil each....