If you buy 10K worth, you'll suddenly buy up every coin between you and some new price.  Most of the market will just watch the spread expand and think "....wtf?"

You'll get some sheep chasing you, but you probably won't start a buying frenzy unless you time it with some real or fabricated "news" that suddenly makes everyone want to buy at any price.

And if you don't do that, you've suddenly got 10K worth of coin that the market's not willing to pay more than 7K for.