Hi Boomer Baby, Rule of thumb on who to add: if the designs appear in the marketplace, they are opted in and you can affiliate them. As for how to add, it depends on what tool you are using to build your affiliate website. Many different options there. Just read the affiliate threads to learn the how-tos. Lots of good info is available to help you out. Also check out the third-party tools thread for information there.
there is no limits on how many products you can add... megastores are built a number of ways, but many are built using the api or their own programming with a database.
most the time there is no way to keep tabs on changes for designs you affiliate except just watching your links. however, the smartest way to build your sites and gain the updated information about new products or price changes, etc. is to build a relationship with the designers you want to affiliate.
happy affiliating!
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