Yes, but only if you have millions of unique visitors a month, and the niche is ludicrous like health/weight loss, personal finance, investing, insurance, etc. Once you gain traction, you can skip the middleman and go directly to the business, like Mint.com, Bankaholic, and InsuranceAgents.com.

If you're trying to sell low margin items like books, and even electronics, it's not sustainable at all. Not even close. Just do back of napkin math and you'll see...