What Ray said. Also you can use Birchwood Casey Super Blue or Brownell's Oxpho-Blue on their website.
http://www.brownells.com/gunsmith-to...-prod1072.aspx
If you want to leave the steel in it's original state then I recommend using carnauba wax and then keep the knife oiled as well as the sheath, but etching is by far the best solution. There is also a product you can buy at most paint stores called Ospho or Jasco Metal Prep, it turns rust from iron oxide into iron phosphate or black iron. It makes a dull finish. I've used it on knives I purposely rusted with salt water brine. It is very long lasting and tough. Works on most any metal as a prep for paint as well. Be careful as it has phosphoric acid in it, use gloves.
I also think Flitz wax is only about $6 a tube if you wanted to give that a try and send to your customers.