Here's how I think about it. Those early users you buy will almost certainly be unprofitable on a unit basis, because you don't yet have product-market fit.

Thus buying users won't scale at first, but it's OK (even recommended) to do things that won't scale early on. http://paulgraham.com/ds.html

So go ahead and buy ads if it helps you get started, but if the ads aren't making a profit right away, think about how to find a path to growth that does scale.