Regarding your second point, I expect that has more to do with the fact that it's difficult to compete with zero-overhead craigslist (and similar sites) for local used goods. There's little reason to use a B&M store to buy or sell such things used.
It still comes down to $$ vs. time. There's a lot of stuff I'd be happy to dump on a store at a theoretical 50 cents on the dollar vs. a private sale rather than dealing with a bunch of individual transactions. I might feel differently once transactions got into the hundreds of dollars but I don't generally want to deal with Craigslist or eBay for selling. Just too much trouble.
Ha, this is kind of the opposite of the ebay issue. I can totally see selling at a store for that reason. I even donate a lot of stuff I'm sure I could sell since it's just easier (and of course someone benefits). But as a buyer, it's going to be much easier to look around online, and most likely cheaper too, buying privately, than trying to buy from a B&M store.
