They have the most knowledge of products, and more importantly, they are able to test and evaluate products, to ensure that they are up to spec. Whatever source you choose, you then have to add price back into the mix. The memory market is a sophisticated one, and if someone is selling at an outrageously low price, there has to be a reason for it. Be wary of deals that are way below what everyone else is charging. Armed with these quotes, you should then approach the memory source you decided on when price was not a factor, and see if they will play ball.
Many manufacturers sell through dealers, as well as selling direct.
What I would do in buying from a manufacturer is to call them directly as see how they wish to sell to you. If they say that they must go through dealers, ask them who their highest volume dealer is in your general region. Then call the dealer and tell him your situation. Be careful about giving them bogus pricing, as these guys know the market very well, and they know if you are lowballing them. In addition, the reality is that they are in business to make money, and there is only so low they can go.