I represent a company which provides ecommerce software and, from our experience, virtually anything sellable in a real world sells online. We've seen hundreds and thousands online store and it's simply impossible to describe the range of goods offered via Internet. Moreover, Web itself is a better place for niche goods, it's just easier for people to find you. Just imagine who are your customers and optimize your content for search engines. And, the more specific you are, the easier it is to find you.
Check out cbengine.com. You can sign up free for 30 days I believe. It lets you quickly see what is currently moving in the marketplace and lets you see things like percentage of refunds, # of affiliates getting commission as well as gravity. Another option is to just watch the news and see where people are having problems, then develop a solution to their problems and sell it to them.
Check out Google trends and compare niches against each other. This is a great way to know what is hot in what countries and cities.
things that no one wants always sells well online like bricks, metal trinket that have no use, and especially vintage clothing that are torn to shreds. keep that in mind
You can sell just about anything online. I would recommend looking at Amazon best sellers and looking at what gets a lot of bids on ebay.
Lol! Don't knock it! People are stupid, look at the popularity and money in ringtones! If you think it's pointless and/or purile, there's probably a huge market! There's one born every minute!