Buying a used higher quality bike vs a lower quality new bike
For the most part, bicycles have terrible resale value, which means that you can usually get a really good deal on a used bike. As for the condition of the components, one can't say without actually seeing them, but quality components should have a much longer life than 1500kms. That said, inspect for obvious signs of wear (there should be almost none), and see if you can find out why the bike is being sold.

As PeterH points out, some parts do need to be replaced with greater frequency, such as brake pads and maybe even the chain and rear cassette. Also, check to see if the bike is stolen with the local police or ask the seller for their original invoice / receipt.

Used really is a great way to get the most bike for your buck.