1. What is the purpose of 'Sell to Market'?
2. I've recently sold some items using 'Sell to Market'. The items became NPC items. However, I never received any credits for the sale. Is this a glitch or did the NPC trader just steal from me?
1. The purpose is to get some small amount of credits for things you no longer have a need for.
2. You should of gotten the credits immediately - check your events as it should of been listed there. If you still don't have it, post to the bug base.