While one can't guarantee people will not undercut prices you can still do something to help you.
Before buying items such as gear and armor, you can check the signature of the crafter in the lower section of the screen, one could ask that crafter if the retainer is his/hers,
when it is not it is the result of either RMT or legit players undercut.
Undercut is not a problem unless prices get too low down to a ridiculous price.