the main page of the TP has 4 columns: highest price, most demanded, most supplied, and one other. all 4 of these are worthless IMO. But besides that, the prices they show are out of date and weirdly calculated: sometimes it seems to take the average of buy and sell price, sometimes just sell price, sometimes just the buy price.
But it’s apparent that some people pay attention to these, as there has been many instances of abuse recently where someone will place 2 million buy orders [for 1c each = 200 gold] for something, just so it will appear on that list so more people will see it and buy it and increase the price. After the person sells all their items, they just remove all the buy offers. This strategy is surprisingly effective and it looks like Anet isn’t really doing anything about it, but maybe I’m wrong. You don’t even need to cheat to make 2 million buy orders = 8000 orders of 250, at 1 order every 2 seconds, that would take 4 hours to place.
For items on the TP, there are 2 key prices: buy order price and sell listing price.
the buy order price is what people who want the item are offering for it
the sell listing price is what people who have the item are selling it for
when you want twilight, you can put in a buy order for it for 2790g and if someone who has it wants to sell it for that price, they can sell it to you and they will instantly get their gold
you can also instantly buy at the sell listing price of 3598g, and you will instantly get your twilight instead of waiting for someone to come along and agree to the lower price
conversely, if you have twilight to sell, you can sell instantly for the buy order price of 2790g or list it at 3598g and wait for someone to come along to buy it.
you can also set any price you want for either of these; it just won’t be filled first and any offers before yours would have to be filled or removed before yours was.
it’s all a question of if you want it now and expensive or later and cheaper.