I would love to be able to give you an answer that would be absolutely correct 100% of the time. But that might not be possible in this particular case.
The best thing to do is to read the Terms of Service or check with the particular merchant.
Some will allow you to make a purchase and receive a commisison for it. However the vast majority don't allow it and will have something written about it in their terms.
As an example, here is what Amazon has to say about it:
Quote:
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You may not purchase products during sessions initiated through the links on your site for your own use, for resale or commercial use of any kind. This includes orders for customers or on behalf of customers or orders for products to be used by you. Such purchases may result (in our sole discretion) in the withholding of referral fees or the termination of this Agreement.
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