Many merchants won't be big enough or interested enough to employ someone to write Adwords ads, so if you make that your area of expertise, there is good money to be made. Certainly a lot less work doing the direct route from Adword ad to merchant, than it is constructing a website and then trying to get the clicks.
Its then just a margin business and the key is to know when increasing your spend isn't generating any more sales.
The other side of it is picking the right merchants. The key is to pick merchants that sell things that you think or know people buy online. Plus find a merchant that looks absolutely credible from their website, makes it easy for visitors to buy, isn't too small and isn't too big (because people will know about them already and go there directly), or is well known but with a difficult to remember web address.
If you're patient, long term (months) tests with low bids will minimise your risk. Clicks still happen down at number 15 and increasingly feedback we've seen is that people are starting to ignore some of the top 'we have everything related to that keyword' type adverts and are looking further down.