The addon collects all the item listings that you seen in the guild store and uploads them to the site. The site then uses all the data collected by all players to generate price info for everyone.
You can also use the website to search for the location of an item using way better filter than the default Guild store GUI. But it is not guarantee the item is still there since low price items tends to get sold fast. You will have to use your own judgment based on the last seen column to predict whether its still available.
Link to download: Tamriel Trade Centre