What should I sell on the Internet?

how to answer to that question, your only choice is research, and lots of it. Based on years of experience, here's the basic process that  successful Online Retailers would go through every time we try to decide on a new product to sell on the Internet.

Find out what the Demand for the product is:

On the Internet is just another place to sell products. The basic concept of Demand is the same on the 'Net as it is anywhere else, and has been for all time. If there aren't enough people who want it, there's no profit in selling it!

Find out what the level of Competition is:

Again, the internet is the same way. The 'Net is just another place to sell things, and if there are too many people selling the same things, nobody makes any money on them.

We start with a dedicated Internet Store shopping site with a high degree of popularity, like Yahoo Shopping, for example. We spend hours in there, acting like a customer, using different search terms to search on the products

Find out what the General Interest level is

 General interest in a product helps to gauge where our Demand and Competition numbers fall into the big picture. For example, if there isn't much Demand for a product, and there isn't much Competition, it would seem that it might not be a good seller. You can't sell something to people if they're not out there looking to buy it. If there aren't many people out there trying to sell it, either, then it's probably not a good idea.

Find out how others are Advertising this product:

 Same thing on the Internet. If you're going to sell a product Online, you're going to have to advertise it in some way or another. Today, Pay Per Click Search Engines are the dominant force in Internet product Advertising.

Once again, we act like a customer. We use as many search terms as we can think of to search for the product we think we want to sell. What we're looking for here is two fold:

 How many other people are paying to Advertise the product Online?

What do their ads look like and say?