Being as this is a webmaster hangout where alot of people own websites and do business online what type of products or services would you most likely buy besides websites/domains?
Promotion tools that work. Stuff not on the list: keyword lists lists of specific types of forums custom marketing research
Looking in the "Buy, Sell, Trade" section will give you a pretty good idea of the services DP readers are looking for. I'd say ll te various forms of link building would be pretty high on the list.