Craigslist: No problems. I've bought a bunch of bikes (ranging from a nice road bike to a mid-grade commuter bike to vintage and beater bikes) on Craigslist. Size of town and time of year can impact supply. Prices for the same item vary widely by location. Advertisements are not always on-point.
Ebay: My favorite road bike came from Ebay, used. Better supply for specific items, though I'm careful about sellers. If it's a larger seller, I'll see if they have a separate website and coupons or shipping deals (which they'll sometimes have, since they're not paying Ebay fees that way.)
FB Marketplace: Seems to be the new Craigslist. I have yet to buy/sell bikes on FB, but have used it for some other large transactions that previously would have been Craigslist territory.