sells advice

If you go to sell it on your retainer, you can see a little "document" icon on the pricing window. Click it to pull up a list of the current prices people are selling the item for. From there, you can click a button to check the history for what people have bought it for in the recent past (and how often it sell).

Now, with this information in hand, do two things: One, check the Resale price on the item tooltip. If the price it's selling at is lower (Or very near) that price, there's little reason to sell on the boards instead of to an NPC. Second, you may wish to look up on a help site how much NPCs sell it for, and for how much. If the common price on the market boards is higher than the price to buy it from an NPC, you can bet most of the savvy customers aren't going to bite. You may still be able to sell, but not as fast as you might like.

Finally, with regards to high quality items (The ones with shiny icons and that symbol by the name), they should be considered different items from the normal quality ones. You don't have to worry about how much NPCs are selling it for (no NPC sells HQ, and folks will pay a hefty price for high quality over normal quality), and you should check the boards for how much people are selling the high-quality versions for, and ignore the normal quality prices.