You can sell items directly to NPC for a set price, or, if it's unused, choose to have it listed on the market at a price you specify. It will stay on the market for one week where other players can view it if they're in the right land and at the right type of merchant. After a week, it disappears unless you manually pull it back off the market yourself before that.
There are typically two sets of merchants in capital cities- one only having items below Q50, and the other everything up to Q100. By traveling to the other towns in the land you'll find merchants with higher Q items.
The selection can sometimes seem slim, but it may largely be to the vast variety of different Q, type, and style gear that's possible. If you can't find what you're looking for to buy, you can look at similar items for sale, identifying their crafter, and try contacting them directly. Or also simply asking in chats (region, faction, guild) for someone who crafts the item can prove fruitful.
This online forum is typically used for high-end top notch gear. That's because the in-game merchants are set price, not auction, along with the fact things disappear in one week if you're not careful- something you'd want to avoid with the best crafts. Many sales here also look for other forms of payment, such as materials or crystals, while merchants are dapper only.
That's right. If you look at your sales on the merchant, next to the item will be written "Resale Time Left 3 days, 5 hours" for example, which tells you the item will disappear in 3 days and 5 hours unless someone buys it or you take it off yourself before then. If the item does disappear, you get a message across your screen saying "Your (item) has been sold, the merchant has already given you the corresponding dapper".
If you want to ask in chats for someone who crafts an item you want, try to be as specific as you can about what you're looking for. Just asking "who can craft shields?" if you're looking for a Q100 shield may lead crafters who could craft you a Q100 shield to not respond because they assume you're looking for Q250, or may lead crafters who can only craft shields up to Q50 to respond. You'll find a suitable crafter a lot faster if you mention the quality you want right away.