I sell auto parts, so that's really all that I'm familiar with. In my experience, if all you're doing is buying the product and selling it you're never going to make any money. For us it's all about the rebates (Master Installer), free direct shipping and annual programs.

The manufacturer sells to their distributors and we buy from them. Theoretically anyone can do this and get the exact same deal. However, we apply for a rebate on everything that we sell. This involves getting the customer registered with the manufacturer and sending off a monthly report of everything that we sell, to whom and for what price. It's time consuming but if you're moving volume it's well worth it. Certainly the only way you can make money and be competitive.

We also take advantage of free direct shipping from the manufacturer. Anything above $3,500 (Blue Sheet) gets you free shipping, though I've seen it as low as $1,500 with other manufacturers. Stock orders coming from the manufacturer are also on a different discount schedule than if we were to purchase out of our distributor's inventory. All orders must still be placed and billed through the distributor though which is its own paperwork nightmare.

Occasionally there's also extra discounts offered on Heavy Duty or Light Duty items which you need to take advantage of.

If you're dealing with a large auto parts manufacturer there will almost never be an option to buy direct, you'll need to go through a distributor. However you need to find out what rebate programs are available. It's easy to do and make money at, you just need to report everything. I would recommend contacting the manufacturers directly to have them set you up with a good distributor and explain their rebating program. If all you're doing is looking at pricing from distributors, you'll never be able to make it work