This can be in any capacity; buy, sell, or browse. I've spent hours just perusing their auctions for nothing but my own enjoyment. I've also used them in a professional sense. When I buy or sell a used firearm I will search for the same item on GunBroker to check the going price. The best way to see what a firearm is 'worth'; is to see how much people are willing to pay for it. The same goes for used accessories too.
This is an open question, please feel free to share your experiences with GunBroker or similar auction sites.