Andrew the easiest sort of thing to create, market and sell is whats called an 'info-product.
Everyone has loads of experience and information stored in their brains, whatever you've done in life you've gained knowledge and experience.
Heres one idea (off the top of my head at 6:45am so it may not be the greatest)...
You're thinking of looking for another job right? OK - thats you and probably millions of other people around the world - new year new job as the saying goes.
So what about a pdf report on say ..
The top 20 difficult questions to have at an interview and how to answer them
20 top tips for dressing to an interview
20 top tips on how to greet your potential employer
20 top - job losing responses to avoid
20 top things to say to impress an employer
.....................
OK so theres 100 questions you'd have to research and provide answers for (I didn't say it would be easy) - but you could interview recruitment agencies. recruiters, personel managers, or create a survey to send out or get filled in online etc etc - spend 6 months compiling the report and refining it (promise them a mention if their question/answer is published and they'll love it)
Then write a 1 page online sales letter for the product, take money automatically online and deliver the product electronically and instantly and hey presto IF done right you've got a massive money machine with a market that will only cease when everybody stops looking for jobs.
The report could sell for say £10 - even an advert in the jobs section of your local paper (thats where your market is) would probably get some sales and maybe enough to advertise nationally.
Just some early morning thoughts
Rob