Normally, we consider Affiliates to be "independent sales reps" who are either selling your product directly, or referring sales leads to your site, in exchange for a commission; thus, our Affiliate programs are designed to operate with that framework in mind. See more info here on how our Affiliate programs work, with explanations of the different Affiliate link types:
http://www.e-junkie.com/bb/topic/2690
NOT RECOMMENDED - UNSUPPORTED HACK - USE ONLY AT YOUR OWN RISK!
If you are adapting our Affiliate program to reimburse your supplier for a product, or to split revenues for a product with someone but not others, this will only work with Affiliate Direct Links used as Buy Now buttons on your site.
This unsupported hack WILL NOT WORK with standard Add to Cart buttons nor Affiliate Hop Links, and you CANNOT have a regular Affiliate program at all if you are doing this; you can however use the non-JavaScript version of Add to Cart buttons for products which have no share percentage set. DO NOT set any common hop link URL or share percentage (avoid using the Admin > Affiliate Program Settings screen at all).
Also, you SHOULD NOT try to use this hack to calculate revenue sharing with multiple different suppliers, as the calculations can become inaccurate if a buyer tries to click a Direct Link for more than one supplier's product. This is because all of the Direct Linked items would get added to the same cart together, but only the last-added item would set which "affiliate" earns commission for ALL items being ordered.
NOT RECOMMENDED - UNSUPPORTED HACK - USE ONLY AT YOUR OWN RISK!
As the Merchant, you would register for the supplier-Affiliate's E-junkie login on their behalf, stay logged-in as them, and click your own Affiliate sign-up link to join their account to your Affiliate program.
Then you would go into their Affiliate Admin and click Get Affiliate Codes, find your business and product to get codes for, and copy their Direct Link codes only for that supplier's own product(s). Then you would paste their Direct Link code into your own site pages, where the link would function as an instant-checkout Buy Now button. The supplier's own Direct Links should be the only way for anyone to purchase that supplier's own products. Every time someone uses that button to purchase one unit of that product, the supplier-Affiliate would get whatever percentage you had configured for them.
Note that you would set a low general commission percentage (e.g. 1%) on that product for all Affiliates, just to enable Affiliate sharing for it at all but without tempting general Affiliates to sell the product with Direct Links of their own. Do not configure any common or product-based Affiliate Hoplink settings at all. Then you would use your Admin > View/Edit Affiliate screen to grant the supplier-Affiliate an extra percentage or per-transaction bonus amount. The extra percentage applies to all links that Affiliate posts, but if their only Affiliate links in use are the Direct Links for their own products, that should work out fine.
Then after the end of each month, you would still need to go into Seller Admin > Pay Your Affiliates to generate the masspay.txt file you would provide to PayPal Mass Payments to pay out any earned commissions.