What are the "Mystery Trading Cards"?

I was in the Steam Market, looking for cheap things to buy/sell as usual, when I noticed this. A "Mystery Trading Card"?

The description is "This card is very mysterious. Very, very mysterious. The kind of mysterious card you might want to hold on to." Will there be some sort of reward if you have this card later?

Solution 1:

On July 12th 2013, the Mystery cards turned into Summer Getaway Trading Cards.

Starting July 3rd, crafting a game badge earned you a "mystery card" rather than a coupon. These mystery cards are now Summer Getaway cards. So if you crafted a badge in the last week, you're already ahead of the game. During the sale, each game badge you craft will get you a Summer Getaway trading card.

Solution 2:

Nobody seems to know what these do yet. You get them for crafting badges, in place of the typical random coupon.

Solution 3:

This is just a guess, as no one knows for sure what mystery cards mean, but:

Will there be some sort of reward if you have this card later?

Yes, probably something to do with Summer Sale, which will begin any day now.

Will it be worth more later?

No. It's a simple law of supply and demand - the more of these cards are on the market, the less they are worth. Price has already dropped from 2,5 to 1,05 Euro.

Only if Steam suddenly stopped mystery card drops (limited supply) and then made them really worth your while (increased demand), then the price would soar. But that's a slim chance, personally I'd bet on the first option, that the price will gradually drop.