These days there are many people making a living, or supplementing their income by buying locally to sell online. It started on Ebay, but it looks like it really exploded when Amazon started their Fulfillment By Amazon program.
Imagine that you find a name brand four pack of facial tissue with a special scent that only Target sells. Target is probably selling these slightly faster than the other scents because it's exclusive, but they will generally sell them at the same price as the non-exclusive scents.
Now imagine there's a guy somewhere who can't get to a Target easily, or just can't be bothered, but loves that brand, and loves that scent. He might be willing to pay two to three times as much as retail to have it shipped to his door. Someone like me could find it on sale, and maybe even stack a couple of other discounts, then sell it on Amazon.
After shipping and fees what I just described are meager profits, but profits none the less. They can even get better when you're looking at clearance items, and the sale of used goods.
Now there are a lot of things that cost money that will potentially increase profits. Things like fancier packaging materials, like vacuum bags for books. Then there are the apps. It seems to me that the ones the professionals use cost around ten dollars a month, and require you to pay Amazon forty for a professional account.
I think I'm going to dip my toe in the water without paying for those kind of extras. Amazon makes a free app that gives me some of that useful data. Other info can be found on websites, which is less convenient. If my only expenses are the items and the shipping costs, I'll almost surely break even, even if I fail miserably.
I am going on vacation for a full month soon, and moving before I do, so this experiment may need to wait until late February. What ever happens, I'll keep you apprised.