You might want to consider offering "multi-domain hosting" as opposed to reseller hosting. So your customer must own all the domains in the account.
This is sort of "half-way" to reseller hosting. It allows the customer to have multiple domains (you could just cap it at 5, 10, 20 domains, whatever you set for that particular plan). But you are in essence getting *cleaner* business IMHO than if you offered reseller hosting. You'll have less chance of spammers, scammers, or resource abusers.
The point is that if you offer reseller hosting, the reseller could conceivably allow anyone to set up on your server, thereby potentially compromising your server. But if your customer is simply someone who runs multiple web properties, there is a much greater chance that the customer will be more responsible with what goes onto your server.
Oh, and regarding the customer service comment, while I appreciate that you prefer to have more experienced users rather than newbies signing up, you really do need to be prepared for the eventuality of having to provide "basic" support to your users. Otherwise, your support business model is flawed.
Good luck.
Vito