What's the point of Mystery Cards on Steam?

Sadly, thats the case with any "virtual" market. Most people are so eager to sell quickly that undercutting goes out of control.
Back when the trading card deal was in beta and the number of users with access was limited the prices were more stable and prone to stay higher.

Once it went open to the public it was a matter of time before each card dropped down to nothing.
Even foils are losing value, my latest foil card was priced exactly the same as the regular variant of its type.

Suffice it tot say for future reference. You're generally best off selling right away as prices are most likely to drop once more people get them and they undercut to sell theirs off.
Its also a clear indication of why Valve will never allow users to sell their steam library contents to other uses in a market fashion as the same would occur. Best hopes we have if courts force this issue is that Valve will "buy back" at a percentage rate for store credit.

I've only heard rumors. Your information is news to me. They were just brought up because clearly nothing related to voting and flash sales this year with the refund system now in place.