gym They should all do it if they want to be in the business. It's impossible to stock every brand and style of gun unless you have 2 million in cash laying around. Many manufacturers have 15 or twenty pistols of one type, like Glock or M&p, and you cannot afford to stock every caliber color and size, "full size, compact and subcompact", in every color combination. Thus you need to be able to get the pistol, or allow the client to buy the pistol from someone else and pay you a fee for transferring it to you.
If you don't do this, you will lose the customer to a store that does.
Some company's have minimum orders in order to become a distributer, like Kimber. You "may" need to put in a twenty to fifty thousand dollar order just to get shipped at a decent price point.The more you buy the more incentives you get, like free guns. really big dealer can get 5-30 free guns on a large order. These guys with 4 or 5 stores buy huge amounts of guns, making it hard for small stores to even get a few guns shipped.
Now just multiply that by the thousands of rifles and pistols and shotguns that people request, and you will understand why you need to allow transfers in your store. You just cannot carry everything that people are going to ask you for.