You can tell when a gear is a quest reward because it will have a small description at the bottom of it related to the quest it was found in.
This is because technically anyone can get the item for free just by doing the quest, so it isn't worth much. If a gear was a reward for a quest, then it's rarely worth more than g regardless of what other features it has. If a gear has a valuable Research trait on it, I'm having troubles determining particular price improvements for traits, there are many traits and they aren't actual items, just stats, so information on their cost is sparse. Normal White gear is largely unaffected by this except in the case of Traits for Research. Certain features of an equipment can increase it's value. Prices in this list may change depending on changes within the game, their availability, and their demand.
Also people in zone chats may sell items at wildly different prices because people on ESO rarely have any idea of what the worth of an item really is. Players may often sell for slightly more or slightly less, it is their choice. Please note that all the prices listed here are considered average and what Overdosed has deemed is the most fair. Forgot your password? This has not been updated in over 6 months, these prices are correct in some ways but incorrect in others.