How do you figure out the market value of whatever you are selling? Is there an add on? In other games you can search the Auction House and see the average price but you cannot do that in ESO with vendors all over the planet.
Tamriel Trade Center addon - show you all listings with minimum, avg, maximum listings and gives estimated price
Master Merchant Addon - Shows you what have sold in your trade guild and at what price it sold (not so helpful if in a small guild)
Apart from these addons you can always check your trade for the item you're planning on selling. And place yourself at the same price or right under (with only a few gold) of that seller.
I usually price something for what I would pay for it or less than what is appearing in traders. A quick search of my quild's trader gives a decent starting point. If there is an item in there that hasn't sold in days, it is probably over-priced. Once I list it, If it sells quickly, the next time I list the same item I raise the price a bit, until it doesn't sell.
The one thing I have learned about selling items in this game is, if you list the item for what it is worth, and it takes weeks to sell, you are losing money. For example, if you have a staff that is worth 100K, and you list it for 100K and it takes 20 days to sell at that price, you are losing money. Since you could have sold that staff for 60K quicker, and used that same trader slot to sell other items in the 20 day period.
On PC, as others have mentioned, there are add ons.
On Console we don't have that luxury, so I simply visit a few guild traders and do a text search for the item I wish to sell, this way I build up a picture of roughly what folk are selling for... I do this for higher priced, rarer items... then I sell mine for similar, often a bit less.
For the other stuff I basically just chuck it in for less than what I would pay.