Eh, I am getting offers that are this sweet. The problem is that the risk is not worth the reward.
If the market goes up, you win. If it goes down, you lose. When people say "over time it goes up," they mean decades (usually 30-year spans, here).
Over a 30-year horizon, this sort of bet is "as safe a gamble as we can guess" when it comes to stock investing.
Over a 1-year horizon, especially at a known high in the markets, speculating on a base of -1% is not a wise thing to do.
Even if you win the gamble, you throw a blight on your credit report for what amounts to a few hundred dollars in gains (with your amount, maybe you could clear 1k, but you could also lose it). Since, in a single year, the stock market can fluctuate significantly up or down, this is not a great idea. Your odds of gain are slightly higher than of loss, but not overwhelmingly higher. If anything happens to you in the interim before June 2018 that prevents you from paying 100% by then, you are set with a surely toxic debt with a skyscraping APR. Two or three weekend shifts or four account opening bonuses will probably get you the same returns with virtually no risk.
