As a Digicert partner, a range of SSL certificates can be purchased directly. These are great for securing your own business website and infrastructure.
To view the product range and to place an order, visit https://www.whmcs.com/ssl-certificates
To resell SSL certificates to your customers in a completely automated manner, use the WHMCS MarketConnect feature, included as standard in WHMCS.
For more information on MarketConnect, visit https://marketplace.whmcs.com/connect