Market Connect is a bad idea. I think you will see that me and others criticized this before it was launched, already in beta. Its nothing but a fancy affiliate system on which WHMCS is reselling your customer services and making some money. Do you seriously want this? After the hard work and investment just give them over your customers?
Basically what the market place is the following:
WHMCS buys the service from a vendor, lets say SpamExperts, they sell it back to you, you sell it back to the customer. You see the problem here?
You can just sign up directly with SpamExperts and skip WHMCS, why would you make your service more expensive and try to pass it trough more hands? Less is better. Less complications.
As for the automation process, nothing is automated. You need to make the changes manually on your server, the automated thing they mentioned is just for selling the service and creating the account.
Its even much worse. For those that are not using MarketConnect we are still forced to have all those files downloaded in our server using space and making the whole WHMCS installation more bloated, even if we are not using it and never will. This means WHMCS is just going to get bigger and bigger as they add new services into MarketConnect. It seems their opt-out is not really opt-out when I see all those files sitting on my server.
Even if this worked completely automated, trust me on this. WHMCS is not going to support your customers in case you have any problems. They barely support us as customers, let alone will they support third party users for services that are not even from them but they are just reselling. My advise is going directly with the vendors, even if its more expensive you have a direct connection and deal with them. You can replicate or copy the whole marketing content from MarketPlace in your website the same way and SpamExperts and more other vendors have WHMCS modules that work directly with your WHMCS account.
The storage thing. WHMCS just copied what SpamExperts offers directly in terms of marketing text, they didn't bother to check if their market connect services works or not with that. You would not have those problems if you had a direct agreement with SpamExperts, even upgrading storage would be a click on your account, their official WHMCS module supports all that. Marketconnect will always be limited to only what WHMCS decided to code in, that means it will be always be inferior to the official module a vendor provides as they can and will most likely support all their services.