When you're sort of "free forming it" ie. trying to start at square one, I generally recommend that people start with the things that they themselves like. Obviously, I don't know what you like, and that may be something that doesn't sell at all - but look at yourself, at your own life, your own interests and passions, and see if you can work outwards from there. The rationale behind this is simply that it is generally difficult to get a serious business off the ground, especially if you are bootstrapping it on limited capital, and while trying to learn the bsiness as you go. So, start with something you like and that holds your interest - it will help keep you from getting discouraged by the learning curve. Plus, if it's something you are really into, you probably already know many of the best places to get whatever the products, accessories, books, etc. for that niche are at retail, so you have a better idea of how to "fan out" in search of wholesalers.
Of course, you might be really into electronics - a lot of people are, which is why it's such a popular niche. If you simply must go into that, be prepared to sell at, near or even slightly below cost, and make your money on upsells, repeat business, accessories, etc. Believe it or not, this is how the major retailers make money on electronics - many of them are selling the main items, the computers and BluRay players and game systems and other items that draw people into the store for virtually zero markup, and are relying on secondary sales to the same customers to make their margins.