Guys, right now keys are at a record high price of 7.44 and the only way they're going to drop is if more get brought into the trading system. The marketplace, which may be cheaper, essentially just recycles keys people have bought from the Mann Co store.
Lately, the marketplace has had an average of $2.48-2.53 from all the times I've seen it. If keys were $2.30 or something, I can understand why so many people would be so keen to buy them there instead of from the Mann Co. Store but $2.48?? Seriously?? You'd rather save 2 cents than buy an original key from the store and help lower its price? And another thing, the more keys you buy, the TF2 cards get dropped for you. You can then resell said TF2 cards for real money or you can craft it into a badge; you'd get more friends and more booster packs which can in turn be sold for extra cash or used to level up your account!
tl;dr buy keys from the Mann Co Store when possible. It's a lot better for the TF2 economy and you save more money in the long run when you sell your TF2 cards for 7-8 cents