Exactly. It's a hobby, but while scaling it up is hard to do, it is not impossible. Two stories.
I wrote a program awhile back that would look for iPod Touches on my local Craigslist and would analyze the price for many different versions. If an individual iPod was available and was below mean price within a given time frame, I'd send the seller a note almost immediately. Then, if I purchased the item, I'd relist it at the mean price. Completely an arb play.
What I learned was that 1) iPods are easy to sell, 2) People who buy on craigslist are flaky and tend not to show up, 3) The time spent waiting around to meet people was obnoxious, and 4) Sometimes sellers see what you did with their item and send you angry emails.
I never made more than a few hundred bucks, but it was an interesting experiment nonetheless.
Second story: For years, my wife sold used books. She'd go to estate auctions and buy boxes and boxes of books. She'd go to library sales and buy hundreds of books. Then she'd sell them on the Amazon marketplace. She would walk into these places with a barcode scanner and would check their price and sales rankings on Amazon. If it was in a decent margin for her, she'd buy the item. This actually did scale, but she quit when it became less of a hobby side income and more like real work. She's a nurse by trade, and the book selling was never meant to be anything more than a hobby.
So, I guess what I'm trying to say is that arbitrage of used goods can be a business, but it depends heavily on the availability of the supply of used goods. Books worked for my wife because she could buy dozens of them at a time and ship dozens of them them all from the post office down the street as orders came in. For me, I was running around town picking up items and dropping them off.