When someone buys your product online, the next step (after ordering) should take them to a page where they can join the online community for the product (or industry).
You should do this whether you're a small design agency (your community might be a mailing list of clients) or whether you're a large online retailer.
Better still, if you know what product they bought (and you really should) you can take them to not only a community landing page, but a page which shows the latest discussions about the product they bought, best tips about the products and most knowledgeable members about the product.
Even if you don't have your own community, you can take them to a page where they can join a relevant online community. Imagine the benefits of flooding an online community with your customers – they can spread the word amongst people that are interested in your products.