So Idk why it took me so long to realize this but I found out how the trading post works recently. When you sell something to the trading post and they offer you a default amount of money that amount is what someone put in a order to pay for that item. By quick selling you're actually just helping the trading post fill orders and if you list the item yourself they'll put your item up for that price for others to buy. I found this out when I tried to sell something I could make for cheap and make a lot of gold for..only for the price I'd get after the first one sold dropped by more than half so the last two I was going to sell went from giving me 98 gold to only 40g so I listed it a gold cheaper than the other high price listing. The item was a thing called a Mystic Fountain Node (or whatever it was exotic quality and parts were dirt cheap) that allows you to access the mystic fountain from anywhere for free and unlike the yellow quality part used to make it it was endless use. Really nice item I hope mine sell but I don't use the fountain that much as is so I don't need a portable one that bad.
On the Wiki page when you see it talk of trading post and prices the top one is what people are currently asking people to pay for them and bottom is what people got orders to buy if you sell one for.