I've bought a few bikes/frames off of eBay and more off of CL. Ebay can be very tough to get a good price and takes a lot of time. Prices often go beyond what they should in the last minute, don't get caught up and pay too much. And check for accuracy, I've seen lots of bikes that don't match up to their description. Look for sleepers, sometimes people will list a bike as local pick up only and if it's close to you, that really narrows the number of bidders and you can save shipping costs and check it out before buying. Last bike I bought off of eBay was five years ago and just sold it for more than double what I paid, though with a lot of upgrades. And I was always buying a second or third bike, so I wasn't in a hurry to buy.
I've seen some great deals on really old bikes on Nextdoor, but selection is very limited.
These days when availability is so low, your best option is going to be local riding groups/networking.
I've got a "Mountain biker" email group at work that I occasionally seem people posting things they want to buy/sell. Most of it is sold within the group, before it ever goes to CL, Facebook Marketplace, whatever.
Also, local mountain biking groups (either your friend group, or larger, like the local mountain biking advocacy group). Lots of time if you put out statements about wanting to buy something, people that have been too busy/lazy/on the fence to list their bike for sale, will respond.
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