If I had any reason to think there may be something wrong mechanically I would ask them if I could go with them to get a mechanic to take a look at it. If they said no and acted suspicious, or didnt let me test ride it, I'd walk away.
Edit: I have seen at least 5-6 bikes the past couple months with less then 2.5k miles selling for $2500, and they had little to no damage. This might not be the same as your area but If I bought a bike with 15k miles I wouldn't pay more than $2200 for it and that's only if it comes with some good mods.
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