I know estate sales and live auctions are where many bargain hunters go, but following those is like a hobby unto itself. eBay isn’t what it used to be. Reverb.com offers very little, although some shops sell new items there. Craigslist seldom lists interesting items in my area.
I believe in supporting local shops whenever possible, and they’re great when it comes time to trade in an instrument. But they have to have margin to operate, so they typically only sell new composite bows or valuable classics. When someone posts “buy a used (insert name) bow” from the last twenty years, I ask myself “from whom?”
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For student/intermediate instruments, it is a bit different but I would bet some stay in families for younger siblings or get placed by a teacher. This happened to my violins. My sister got a few-decades-old viola made in the 50s. Her piano teacher was also a cellist in a local orchestra, and when one of the violists died she passed the word to his widow that there might be a buyer. I have it now.
Full disclosure: I’m interested in selling a bow and also buying a professional bow from someone who has moved on from theirs.
I concede Stephen's point that many instruments stay in the family (or perhaps go to the attic). My teacher, who is a professional symphony musician, reports that his colleagues regularly show up to practice with different bows/chinrests/whatnot. So there's turnover and churn. I just don't see that reflected on the internet.
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They’re not good places for anyone who isn’t able to spot forgeries and bad repairs, but there are several auction houses and online auction platforms dedicated to the sale of old violins. While Maestronet and Violinist.com don’t have marketplaces, the Fiddle Hangout site does have one.
Craigslist and Facebook marketplace sometimes have instruments listed, but they are similar to estate sales—the items, condition, and prices are all over the place and it’s easy to get burned. eBay seems to be the worst place to find fiddles, as there are innumerable scams and dodges at both ends of the transaction—at least if you’re using the other platforms, you’re meeting someone in person before purchasing and have a chance to inspect the item quickly. Caveat emptor.
If you’re looking at the lower end of the market, the options tend to be new factory instruments because those offer the highest markup for the shops that sell them and they’re so readily available.