RE: The Fallacy of Ebay, Craigslist, Poshmark, etc.
I've used eBay for both buying and selling a lot over the years. It's great if you have old stuff you just want to get rid of. Weight it, pack it, and list it at a starting price of $0.99 (plus actual shipping costs) and you are done. You might only make $0.99 or you might make a lot more depending on what it is. The point is you've traded junk you don't want for some cash.
If you are talking about buying and reselling, then yeah of course it's a business and you have to know what things are worth. I don't do that too often unless I just happen to run across something I know I can flip for a profit. For example, a couple years ago I found some 1970s era Dungeons and Dragons books for $10 at a yard sale. I didn't know how much they were worth but I knew it was more than $10. I think they sold for a little over $100 on eBay.
I don't like how eBay has a virtual monopoly and the standard online auction thing. There used to be yahoo auctions back in the day that I used to some success but sadly that ceased operations long ago. Some more decent competition would help bring those fees down.
Yeah, they pretty much got a monopoly. There are a few other sites, but they don't really get much traffic. Even Ebay doesn't really get much traffic. At least they don't get enough traffic to overcome the sheer amount of pure garbage that's miscategorized on there. There's just so much crap on there that you really gotta go searching, even when you know exactly what you want. When it comes to having someone find your listing, you gotta be lucky, and they gotta be determined.
I wish they hadn't let Yahoo just got to crap. It was a pretty alright company back in the day. If they had kept improving everything over the years, they would likely be a competitor to Google and others, rather than just the place where you mom gets her email. They basically lost all their value due to complete incompetence.