Because so many gun dealers work across state lines, the federal government must create legislation to make sure that dealers are not taking advantage of one state's gun laws at the expense of another. Studies have shown that guns can be bought in states with lax gun laws, and then smuggled into states with strict gun laws, bypassing that state's laws. Federal laws could even the playing field, so to say.
Some federal laws are needed for gun dealers just to make sure the dealers are honestly conducting their business. If there are no present laws, then gun dealers might get the idea that they can sell guns for profit to customers they are not allowed to sell. They might be motivated to make wrong choices in the name of profit.
Gun dealers should be subject to the same gun laws as anyone else. A supplier of weapons to the public should need to follow certain guidelines for public safety. What these guidelines are is a subject of great debate which I will not get into, but there is no reason why the main source of firearms to the public should not follow federal laws.
I do not think that we need federal laws for gun dealers. I think that each state should be able to govern and dictated how guns and the sales of them are dealt with. To impose a federal law among all gun dealers would be an unjust and unfair policy.