"The irony here is that when you factor in the mailing cost (and E-bay fees) you're better off at the local flea market, or holding a garage sale."
That depends entirely on what you're selling. Me, I've recently started buying defunct outboard motors with either collector value or more modern ones. I strip 'em and sell the parts on eBay. You can't do that at a flea market, you see. There are no customers. On eBay, the guy looking for a cylinder head for a 1923 Johnson outboard motor is gonna bid on yours, since it's the only game in town.
On eBay, the most obscure item will find bidders. In fact, you can sell almost anything unusual on eBay for WAY more than you'd ever get for it at a flea market or garage sale.
eBay has revolutionized the collectible and antiques business.
You can't sell antiques at a garage sale and get a fair price for them. If you send them to the local auction house here, the commission fee is 20%. I ran an antique booth at a local mall for several years and besides my rent, I had to pay a commision of 10-20% depending on the cost of the item.
I have shopped for car parts. It even got me to consider buying junk cars and selling them part by part. Then sell the remaining hulk for scrap metal.
I would much rather pay $25, part and shipping, for that power window switch on eBay than go running all over town digging in junk yards.
And yeah, the USPS deal is a good one. I got new boxes just last week.
I recently sold a book on eBay for several thousand dollars (it was salvaged from my father's attic). I don't think I would have gotten that at a yard sale or flea market.