Many Affiliates will assume they can use their commission to get a "rebate" on the item as a sort of "wholesale-priced" sample, since they'll be selling the product for you and would want to know exactly what they're selling. They may also attempt a test purchase via their own link to verify that they will be credited for their sales conversions, rather than risking their time and effort to promote products on promises of commission alone; they may want to see how you "walk your talk" first.
If you don't want to honor any commission where the Affiliate themselves was their sole buyer, it's a good idea to be clear about that on your Affiliate terms or recruitment page, at least implicitly by saying they will earn commission on purchases that others make after following the Affiliate's link (key word here: "others" :^). That still leaves you the option of giving productive Affiliates a "rebate" commission on their own order if they did actually generate real sales.
Although you can choose to pay your Affiliates by whatever manner and schedule you wish, we support and recommend making monthly lump-sum payments using PayPal Mass Payments. After the end of each calendar month, you have until the end of the following month to use your Seller Admin > Pay Your Affiliates link to obtain the Mass Payments file for the completed month and follow the procedure to pay your Affiliates. You can also adjust payment amounts manually if you wish by editing the payment file -- this page describes the format: