What is the percentage that my product will sell?

I think the most important question to ask yourself is how am I providing unique value? Try to think like a customer. Why would you buy this product. Testimonails work great. And do not forget the follow up system. People will not buy right away. You need to have an autoresponder system which will keep bringing them back to your site.

If I may say so, I think this is a better question that the ones that started off the thread.

It depends partly on how you're going to sell whatever it is - how you're going to attract traffic. For example, if you're going to try article marketing, it needs to be something you can write about.

The best things to sell are things that (a) people already want to buy (so you don't need to persuade them for ever), and (b) people want to buy quickly because they see them as a possible solution to a pressing problem, and (c) things that not too many other people are selling (so that the competition doesn't kill you, and the "entry-barriers" to the business aren't set too high for you).

It's a tricky balance, because the third of those considerations is mismatched with the first two.

And of course it depends on whether you want to be a vendor or an affiliate, because if you want to be an affiliate, you have to choose from what's already available in the market and can't create something entirely new of your own. For myself, I'm an affiliate only (for now) and I aim to sell things - mostly but not entirely from Clickbank - for which there isn't overwhelming competition ... because I like a fast start when I start off a project, and I've successfully built up 8 separate, unrelated niches that way, which I think also minimises some of my risks. Being an affiliate has many advantages over being a vendor, in my opinion. It's easy to switch products relatively quickly, easy to be flexible, easy to avoid customer service altogether, and so on. One develops, at low risk, a secure, asset-based (niche sites and lists) business free from most of the problems which vendors have. Some will disagree.