I'd respectfully disagree. The original context of Cuban's advice is here:
http://blogmaverick.com/2012/03/18/dont-follow-your-passion-...
..and an especially relevant excerpt related to your comment:
"Why? Because everyone is passionate about something... ... [things] that we really really want to do with our lives. Those passions aren’t worth a nickel."
His overall point is that instead of blindly following after your passions and "hoping" things will work out, you should count on effort and hard work.
Technically, you're right - Cuban wasn't talking about specifically picking an eCommerce niche. But he does make the great point that if you're successful at something - regardless of what it is - you usually become passionate about it.
The problem faced by many new entrepreneurs is that their passion-based business plan doesn't get traction, and they give up. But if you start with a effort and market based business plan, a new entrepreneur is much more likely to succeed. And when they're successful with their business, they'll become very passionate about it.