Probably 50 years ago I asked my dad for advice about how to price something I was trying to sell. He told me to just pick a price. A month from now if I still had it, my price was too high. If I had sold it, my price was too low.
I think that works for buying things like used bikes too: Establish in your own mind what you think each bike ought to be worth. A month from now, if you're still looking, your value judgement is unrealistically low. If you've bought a bike, your judgment was too high.
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My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.