Generally, if it’s related to something you can buy from Amazon.com, it’s a decent topic for a blog. Look around that website – if you’ve ever bought something from Amazon, they’re probably begging you to buy more of it. Ask yourself if that might make an interesting niche.
Normally, you’d need to dig very deeply into the site to find things that are very specific and then drill down to something more profitable – but I’ve got a (semi) automated way of dealing with that when it comes. So, just look around for high-level ideas.
Don’t pick things that sell at a very low price point – you don’t want to throw yourself into hard work for a tiny 4% commission on a 2 dollar pen. Remember the Amazon’s commission only goes up to 8.5%, but the commission rate increases with the more products you sell. It’s easier to get that higher commission rate once you’ve got 4-5 sites chipping in. Look for stuff that costs at least a hundred dollars. YES, people buy expensive stuff from Amazon all the time!
Look for accessories: smaller items people might buy in a bundle along with that more expensive product. For example, if someone buys cookware, they're likely to pick up a book of recipes to go along with it.
Make sure there's a market with this product. The idea is that you want to promote a product that has competing products. That way you can build trust by comparing them to each other.
Don’t pick obscure items that don’t have any reviews. I try to look for products that have at least 4-5 reviews that I can quote from and point too. This also helps the buyer feel comfortable in their purchase (remember, you don’t get paid unless they order).
Once you’ve done that, take a look at Amazon’s bestsellers for different categories. Keep an eye on those products and products those buyers also bought. Grab a pen and pencil, or open a file in Notepad. Spend a little time and write down the things that stick out to you. They don’t need to all be related, just do some careful research and cast a net.
There are lots of “no duh” physical products out there that people buy all the time from stores.
Embroidery machines, lawnmowers, bread machines, microwaves, strollers… just use your head.