Price shopping for a gun (or many other readily available items), and price shopping to have a deck built (just an example) is not the same thing.
When buying a gun (for example), it's an item that will be exactly the same no matter where you buy it, unless it's messed up from the factory. If I buy an XD40 (for example) at Jim Bob's Gun Tavern, it will be the exact same product that I could buy at countless other retailers. there will be almost no chance of quality differences. If something goes wrong with the gun, it would likely be the responsibility of the manufacturer, not Jim Bob's Gun Tavern.
On the other hand--If I were having a deck built (again, for example), I wouldn't necessarily look for the cheapest price. Reputation, referrals, BBB reports, ect. would be just a few other considerations worth looking into. Afterall, the person building the deck will be responsible if anything were to go wrong.