This is how I originally started with my online money making methods. They key here, just as the op said, is to find "good products" or just anything that is in demand at the time. By applying a few simple tricks, you can easily scale this up into a full-time work from home business earning you an extra $500-1000/mo or even more. I personally know a few people that make over $10,000 per year doing just this.
Clearance items are a great place to start because the stores that sell them at the reduced priced have made their revenue already (inventory turn). I used to install car stereos and had lots of leftover products in my shop...things such as speaker grills, radio harnesses, or even old factory speakers. As technology changes, older products become outdated but their manufacturer or vehicle specific parts stay in demand. I would take these items and sell on eBay (I manage between 3-5 seller accounts in different niches) or send them in a box to Amazon (Amazon FBA does this) and they would handle all the shipping and returns for me. All you need to do is create the listing and pay a small warehouse fee.
Used/new books are also an easy target right now. People are stuck at home because of the pandemic and looking for things to do in their spare time. They've watched all the Netflix shows and listened to all the podcasts so they are turning to books. You can buy dozens of used books for around 50 cents to a dollar and ship them to Amazon to sell. Amazon has the listing in their system by ISBN number so all you need to do is create a used book price. I did this several years ago with used college textbooks and made several hundred a month extra.
Biggest key is to find your market like flixxx2 mentions. I recommend selecting products in an area of interest for you. One of my eBay seller accounts and Amazon FBA accounts sold model rocketry items. I would buy boxes of rocket engines, launch pads, factory sealed rocket kits, etc from estate sales or bulk lots on eBay. Then, I would keeps the items I wanted for myself (hobby of mine) and sell the rest individually or in small lots. This allowed me to fund my hobby AND make me money at the same time! To scale this up, you could create a website selling these items directly, create or join a local hobbyist group to sell to, or outsource that work to someone else (think Fiverr) and focus on other projects.
Right now, I've been getting back into crypto mining as a hobby/business and working on branding/marketing. With the price or new/used video cards and demand being so high, there is plenty of opportunity for making money in this area of interest for me.