I don't know if you guys noticed, but Flippa is a great way to find undeveloped niches! I check it out every know and then looking for retail sites selling unusual merchandise... Per example, there's a site right know that is selling hammocks... As silly as it looks, I'm pretty sure there's a penny to be made selling hammocks online
There can be a niche in almost everything. There will always be people interested in products to sell, you just have to find the right one.
Look, I know this will probably fall on deaf ears, but if you want to know about "profitable niches" then pick up a book called "Micro Trends" by Mark J Penn. This is the guy that created the term "Soccer Mum" for Clintons first campaign.
Be careful with flippa. Black hat marketers created millions of micro sites, get fake traffic, and raise all the stats just to "flip" the website after a few months. Ebay and Amazon I think are much better.
OP, thank you for your post and attempt to help the community at large. Zurpit here has a point though.
Here's another way to find great niches - use the Google search autocomplete. For example, type the beginning of a question in such as "How can I stop " Google will give you a list of search suggestions. In this case it includes: snoring, biting my nails, being jealous, sweating and so on. Right there is a list of niches you're guaranteed are in demand. Autocomplete suggestions only ever come up if there is enough search frequency for the phrase. You can monetize these niches easily with info products or affiliate offers.