I have never used MArket Connect to sell SSL, as I have always done it via the ENOM module.
Is there any advantage to doing it via market connect?
Instead of reselling from Enom you would be reselling from WHMCS. That is the main difference. The biggest issue I see is that WHMCS can't even properly support its own software and now they plant support third party services. Ouch...
Market Connect is a glorified affiliate system. WHMCS is trying to make money by reselling things that most of its customers already sell. To me this is also a huge conflict of interests. Your customers are now WHMCS customers.