Minion – Addon Manager
A must have app, makes managing and looking for addons very easy, get it HERE
Othewise, you will have to download and install manualy from ESOUI.
Arkadius Trade Tools (ATT)
Basicly, it does same as Master Merchant (MM), just with much less impact on resources. It scans guild store sales records and generates average price. Also, it can be used for tracking your guild sales/purchases. I would never recommend any of those alone to define your price. ATT and MM can easily be manipulated. You should use it combined with TTC at least.
Tamriel Trade Centre (TTC)
Another addon for item pricing, however, it functions differently than ATT and MM. TTC doesent rely on sales (only), it also has 3rd party app, which gather all item listing data to website. So, you dont get only sales prices from guilds you’re in, you get all listing data from all people which use TTC. Using TTC with other pricing addons is highly recommended (you will get most accurate price info).