gunbroker.com is an efficient way to get buyers and sellers together to buy/sell stuff at a price they both agree to. I'm all good with that part of the transaction. That's 100% legit.
The easy money you can make on gunbroker.com makes it easy to buy low and sell high, if you have the time, skills, or software to get the deals. With so many people able to make a quick buck, it can make it harder to find inventory on the shelves, or even online. I think there's enough of that going on to make it seem like there's even less supply then there really is. I think that's where gunbroker.com influences the regular supply and demand model. There's an extra middleman (100's or 1,000's of them) that's inserted themselves into the supply chain and it makes things less efficient. It's not just gunbroker.com. It's gunbroker.com, plus multiple other online sites, local online classifieds. Unfortunately, it's made it's way to this site as well. It's very likely these scalpers aren't legitimate, licensed, or insured businesses. They aren't paying taxes on their gains. It's very disruptive.