Yes I did that before for a local real estate company and it worked our really good for both of us. They were paying $15 per click and converting at 4% so they weren't making that much in profits. I approached them and they accepted, I ended up accounting for nearly half of their sales and profits went up 400% even with me getting 40% per sale.

I think the best way to approach this and the way I did it is to write them an email first with info about you, your knowledge of the market/niche, maybe some examples of your current seo/adwords work and then put your phone number at the end so they can give you a call. A lot of online businesses, especially the new ones, are worried about getting scammed on ad sales or partnerships from unknown people so make sure you proof read your offer so that it doesn't sound like you want money up front or ownership/rights over any of their content, products, services or any access to their website/customers.