If the bike has been well maintained and things like the chain and gears replaced when they are worn out there is no problem with buying a high milage bike. You will want to ask how often, and when the last service was.
The higher mileage, without any service, the more likely that components will need to be replaced sooner. I have put over 4000km on my RANS Rocket with regular servicing, and while it now needs a new chain, it is still good for many more kilometers yet.
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Get a bicycle. You will not regret it if you live. ~Mark Twain, "Taming the Bicycle"
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