On eBay it costs money to raise the starting bid above zero. I don't know if GunBroker is the same way. But that could be one reason why.
I personally start an auction at the lowest price I'm willing to accept. But I don't expect others to do so. As a seller I don't care if someone misses out on other auctions. I care about my auctions only. And this is what you have to realize as a buyer.