I don't know how they do it, but they do it well. Someone here once said that they "buy like wholesalers and sell like retailers". To me, that would mean that they buy in large quantities to get bulk prices, but then they sell them off one by one to the public. This gives them the ability to price their guns better than most anyone else. I know they operate on a thin margin and rely on the mass of sales to generate their profit. The Walmart approach, if you will.
I've spoken to their staff a few times and asked them how much a particular gun would be once they got it in stock. He couldn't tell me because, he said, they don't know what price they will be able to get those guns for, and their selling price is based on the price they pay. That has to mean that they adhere to a strict margin.
I've bought a number of guns from Bud's. They lack nothing IMHO.