Depends on the seller I think. If you know a lot about electronics and believe you have exciting products, why not? On the other hand, if you have a totally unique idea you may be better off as it will be new, fun, dangerous and exciting. A few of the main characteristics of an entreprenuer.
There is no right or wrong answer to this question. It all depends on people's own experiences of selling each and the circumstances they were in at the time. Statistically you will more than likely find that unbranded products and those that aren't so competitive (consumer electronics for instance) will present the highest per product margins.
I think this is a question you should be asking yourself and perhaps your real focus should be on what you are doing to find your own answer rather than gauging what others are experiencing.
Tough one...it really depends on the individual and the way that you do business. I would suggest doing some research into the all the areas you have highlighted above and drawing up your own conclusion on what may be best for you.
The thing i find is don't get confused between "mainstream" & "profitable"... Games & Consoles are the perfect example, whilst probably the most popular genre - you will find profits very small per game & competition very high, therefore you have to sell a good volume to make a worth while return. You'll be suprised at what actually does generate good profit (my dead fred props during halloween for example - only 2 of us were selling them on eBay - wish i had purchased more! lol).