I sell 10-20 things a week. 95% are great and hassle free. The other 5% are eventually fine as well although it takes a bit of work. Run it like a store. Back your stuff, accept it if people want to return.
It's about getting rid of junk, not about making money. You'll come out ahead, not by a lot, but ahead.
Some tips:
Amazon boxes are lighter than "normal" cardboard boxes.
Buy padded envelopes in bulk.
Obo is usually a waste of time. "It's marked as "let buyers make offers" --- you get mostly lowballing hacks looking for over 50% off the market rate
Given that, counter-intuitively, pricing things too cheap can scare people away. Raising prices can sometimes lead to faster sales
"Buy it now" usually has better customers than auctions. I can't explain the psychology but I've sold a couple hundred of each, it's clear.
Don't worry about the limits if you're putting on high quality stuff, they are to prevent someone from posting for example, 10,000 nearly identical phone case variations and flooding the site with crap. You'll get constant "promotions" if you push enough volume
Sometimes your pictures work better, sometimes the white background professional ones you just lift from Google images does. I can't explain this one yet but some things sit idle for weeks and then I switch out the image (going either way) and they sell out in a few hours. It's weird.
Usps is pretty lenient within a margin of error. If you accidentally send, say, a $7.87 package as a Regional Box A that costs $7.49, they seem to not care. A 12.1 oz package sent as a 12oz? They also don't care. I've caught a few mistakes I made and waited and watched the package, everything went smoothly. Also, FedEx can be cheaper for larger items, don't forget to check that tab. They're open pretty late and it's pretty easy
For calculations roughly take .87 * sale - shipping cost - $.50 for packaging.
You may find out you only make $5 from that PCI ATA controller you had in that box but now it's finally gone, going to a good home, and hey, look at that, you just got $5.
Is it a waste of time? Well that's entirely up to you to decide. I looked at it as a hobby like gardening or crafts. As far as hobbies go, profitable ones are rare. Also I really have too much stuff.
Also, if you ever wanted to try marketing and sales, the business side of tech, this is a nice soft introduction, a very low cost, low risk way to gain an intuition on pricing, place, product, pitch, etc.
Ebay: A++++ highly recommended, would sell again.