6000 miles is nothing for a frame. It's just not a factor.

A well-maintained road bike with 6k miles on it should be...

...on its second or possibly even third set of tires, depending on the tires.

...on its second or third chain.

...on its original cassette, which should be checked carefully for wear, especially if it's only the bike's second chain.

...on original parts for the rest of the bike, unbless something was upgraded by coice (in which case ask why).

If the wheels are true and have mostly the original spokes, chances are it was well-built and will last many more thousands of miles.

If the bike fits you it could be an excellent buy. If it doesn't fit you you're wasting your money. I can't tell you how many times I've seen people buy a poorly-fitting bike because it seemed like a bargain and they had a bad case of "new toy fever," and end up with something they couldn't ride.

RichC