When you click on an item in your retainer's inventory, it already brings up a list of options. I don't see any issue with adding a "Sell" option to that list, which will cause the item to be cashed in for its NPC-price value.

It's not a huge deal, but I can see the value in it when it comes to housecleaning.

"Bah, my retainer is full of junk. Let's see what I can get rid of... I can marketboard those, but THESE aren't even selling for NPC price, so I'll just NPC them. There's twenty, though, and I only have four slot left in my inventory. Bah, guess I have to swap out items from my inventory for the stuff I wanna sell, and then come back and load up my inventory again. What a pain! Guess that's what I get for leveling FSH..."

If only there was a way to NPC-sell directly from your retainer, without first putting it into your own inventory!