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

I would assume that what MC means by "protectable" is whether or not you can obtain a patent for it. To do that, you would need to have a patentability search in order to determine if your idea is patentable - are there patents out there which are similar to your idea and how similar to your idea those patents are which would of course mean that even if you got a patent, how valuable would it be, is it effectively "groundbreaking" or is it an improvement and how much is it improving the technology. Consult a patent professional.

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.

There are many ways a business can create a defensible moat (as Warren Buffet puts it). It can be the fanaticism of the fanbase (Bob Dylan etc), could be network effects, could be regulatory burdens on competitors that you are exempt from (Indian reservations) or even a unique location (NZ no software patents). But note that most IP provisions are intended to be used as shield (against rip-off, passing off, leapfrog of sunk costs) and not sword. Only patents have that position and software patents are not trivial to gain. There are tactical plays you can try depending on the business (Indian rope trick), or the stairwell method that Telsa uses in continually dropping prices (economies of scale) so nobody except well-financed companies can enter. But I'd just note that most network B2C activities have pretty much been occupied (see Union Square investment thesis 2.0) so you have to work a bit harder like unique proprietary tech or ultra-loyal DevOps willing to go extra mile.