My sister does this. She makes a few thousand a month selling stuff on ebay that she picks out of the bins at Goodwill in Portland.
I went up there for Christmas to help automate her workflow. She was doing it on Gen 1 Surface which was less than ideal. So I gave her my old computer. She actually likes digging through the bins looking for gems. It is just listing them on eBay that is pain since she has to wash the stuff and then take pictures and write up a description and list things.
She has a good eye. I went to the bins with with her and she would pull out a pair of yoga pants and say should could get 60 bucks for them. And she did.
So there is a bit of skill involved. But she consistently makes a nice chunk of change and I would say about 95% of what she lists get sold.
As a buyer I found that the key is to find a seller who specialises in items that were originally good quality and high ticket prices. For example there's a UK company called Rohan who make outdoor/adventure/travel clothing, it's great gear but eye watering prices. However on Ebay it's relatively easy to find sellers of this gear and, because the stuff is good quality, older items are still perfectly servicable ( even waterproofs ). Also I think that the better quality manufacturers will have more consistent sizing which makes buying easier.
Personally I prefer Rohan's older designs as I think that they're more practical which is even better because they're cheaper! These days all my walking gear, other than boots, comes from Ebay.
Aha, so another case of arbitrage between old-school brick-and-mortar merchants and Internet sales. People make their living visiting mom-and-pop stamp dealers (or coin dealers, or used bookstores), run by old guys who don't get the "Internet" thing, buy up suitable inventory, and re-sell on the web. It works especially well when, for example, a stamp dealer in small town U.S.A. has a stock of French stamps (French coins, rare French books). Locals aren't interested in French stamps, but if you sell on the Internet, you can find buyers in France.
