its true, no one can tell you whats good to sell, you have to do your own research. Cos what if I told you it was good to sell a high selling product like mp4 players? then you go out and buy them and in the end find out youre losing more money than making because of faulty items, customs blah blah blah. I know a few people who fell in that trap, went for the popular item that was high demand, heaps of sellers selling em and they lost more money than made.
So you have to do your research and find a product that has a small amount of competitors but high in demand. When you have done that, then even more research comes in... yip you have to find a reputable supplier for the product who sells at low prices.
It can take lots of time to do all this (not always the case, but it was in my case anyway) but Im telling you its all worth it, the hard work will pay off.