I am evaluating a used Trek OCLV bike from 2003. Seller did a rebuild with lots of new Ultegra components (STI shifters, SRAM 10-speed cassette, etc.). A friend told me the price is too good to be true (~$1100), but seller insists its never been in an accident and the bike rides really really nice. It seems nicer than other new (aluminum) bikes in this price range that I am considering. I couldn't find any visual damage on the frame besides some light scratches near the crank.

Normally I would shy away from something too good to be true, but seller seems knowledgeable and legit (e.g., lives in a nice neighborhood, has lots of toys, he and wife ride nice new Trek Madone bikes).

But there is one issue: on a hard brake, the fork shuddered quite a bit. Seller says he will check it out and redue/tighten the headset.

Any ideas on the fork shuddering?

Any other red flags I should be aware of when considering a used carbon fiber bike?