Well as I said, I'm in the research phase, searching and narrowing down what it is that I'll put on my store. I discover an item that looks good on a wholesale site, then check that exact item type on Amazon to see how popular it is, how many are already on there selling said item, and if they are, whether I could do the same product at a more competitive price. Popular items are usually the more saturated, which can be worth a punt if the market is big enough. Ideally you'll find a more niche but in-demand product or product range.

I don't really know what more I can say in terms of describing my product selection at this early stage.

Amazon have a calculator on their site you can use to input the required figures in terms of costs, item size/s, monthly fees etc etc. I haven't yet actually used it because right now I'm unable to spend all my free time doing this (it's probably going to be a ball-ache working out how much storage space you'll require in their warehouse).