@Kathey,
I think you already got a good answer for sorting WW (by region, etc).
This assumes you only collect stamps (not covers, etc) and that you are not going to delve into varieties, printing versions, etc of any particular issue.
As I collect only Canada, I still have too many categories to do them all justice, but I keep adding to them all (mainly through bulk purchases from professional stamp auctions and
ebay.)
I am careful to examine a lot the best I can before bidding to know what I can expect to get back on partial re-sale. Sometimes I will bid on something not well illustrated/described and hope for a find, as these often go for less.
I have begun to purchase single stamps to fill in spots in my main Canada collection as I only need the earlies (I like used more than mint for these, partly due the price difference, but also because they have a history of actually being used for the purpose they were intended - must be a nice cancel and well-centered though.)
As far as making back on re-sale, the key here is knowledge. I have spent money on books that deal with Canadian stamps in order to be able to sell knowledgeably to collectors that are very specific on what they are looking for (eg.paper, plate and perforation varieties.)
It is more than worthwhile as a seller, but I enjoy the research, and not everyone wants to delve that deeply into a specialty - I have done so with at least a half dozen areas of Canadian philately.
I wouldn't do it just for the selling aspect, I enjoy this type of in depth research - it is my hobby. It does help in my selling tremendously though.
I sell through an
ebay store - "Rehautala". - hope I'm allowed to mention that.
Hope that helps.