Definitely stick with something you know. It will make it easier to know what is hot in that niche and what people are looking for. It also helps with customer service when someone calls or emails to be able to talk to them first hand about the product you are selling. A good niche is also one that has very few people in the same niche, which is hard these days on the web. Make sure it is not to broad a category, start small selling just the blue small widgets then branch out to the larger ones and then the red and yellow widgets.