Ahhh, this is the fun bit lol
Out of all the words involved in describing online sales, RESEARCH is the most important!
It all comes down to research, research and more research. In fat, just when you think you've done enough research, it's only just starting. Research gets you moving forward in this game, it keeps you i the game once you are moving forward and maps out the future direction you head in.
You're right, it isn't about what you like personally, it's about where there is a market. That said, you have to start somewhere and I think starting your research based on products that you do like is as good of a place as any. Select your marketplace, look for sellers within that market place to id your competition.
You can use the research lab to get statistical data on products, so that should be useful for you. It's a scaled down version of Terpeak, so Terapeak is always an option you may want to consider if you think you need to drill down deeper on that type of data, but it does cost, where as our tool is free.