I use it a lot lot lot. I play TEC, so I usually have credits more available than resources, especially when I get Trade Ports going everywhere. So I tend to buy resources when I need them, especially when I'm in a research frenzy, or in a rapid colonisation phase.

In games where I have an imbalance of resources, usually producing a lot more metal than crystal, I sell metal and buy crystal.

The object is to get things built and researched asap, so waiting around for one resourse when you have an oversupply of another is a waste of time, that you may pay for later. Just be careful though not to oversell one to get another.

Nothing really special about them. Using those buttons lists a chunk of your resources as available for sale and it shows you the price (which will be higher than the instant sell price). When AI (or players) click the buy buttons on their end, your stuff gets sold. If you're playing with other humans, you can lower the price of your listing to make sure your stuff gets bought first, since whoever hits buy automatically buys the lowest priced available. The AI doesn't compete with you here, so you never have to mess with the prices at all. If you don't need instant money, you can just list a bunch of stuff and it'll get sold eventually, netting you many more credits than using instant sell.