In addition to the affiliate marketing stuff I do with eBay (ePN) I’m also pretty involved in two other niches that I’ve promoted with both SEO methods and PPC. For the latter two niches it has been a lesson in shifting economic trends as well as what areas seems to resist economic downturns.
I won’t discuss how to do market research since this is a complex topic that gets discussed to death everywhere. It’s certainly a very important topic and it’s one of the first things newbies get initiated into when first venturing online.