How do we know about the protectability of an "idea"?

Ideas, in isolation, are not protectable. Reducing an idea to practice (i.e., as an invention) may be protected under a patent if it is sufficiently new, useful, and non-obvious over the prior art; generally, preexisting and known technology. Further, an original expression of an idea may be protected under copyright.

To what Mr. Cuban is mostly referring is whether or not what the company is doing / selling is protectable under patent; i.e., can the company claim a quasi-monopoly over the exploitation of the product or service. Determining whether your technology is patentable in the U.S. would be accomplished by performing a prior art search and evaluating the results under Sections 102 and 103 of the U.S. Patent Act.

Of course, there would likely be some copyright-protectable features of your "online B2C platform" and you may protect your own brand by obtaining a trademark registration, but an investor would likely be more concerned with the patentability question, if not expecting a patent application to have already been filed or (even better) a patent issued, regardless of how enforceable the patent might be in this legal climate. Further, there may be preexisting patent rights to technology you are considering using, selling, offering to sell, etc., which may pose a litigious risk.

It would be advisable to speak with a patent attorney to discuss the patentability of your technology, bars to that patentability (which may be in the form of enforceable patents create an infringement risk), and how best to protect your larger enterprise.