I am about to buy a used 5d classic. Seller told there are some hairlinecracks on body. Is this important. If so, to what extend?
Depends on where the cracks are, if they allow light into the camera and what kind of conditions you plan to be shooting in. If they are on more cosmetic parts, it probably doesn't matter at all. They may eventually be failure points, but if they are on replaceable parts, it doesn't matter too much.
I'd certainly ask for photos of the cracks as well as some photos taken with the camera in light environments, dark environments and with the lens cap on in a bright environment.
Watch out: the 5D is not weather sealed, but as it is, it an catch raindrops from some light rain and survive. Now, if you have cracks, if they are sufficiently deep, water can get in the camera, dust can get in as well, etc.
Differentiate between cracks that start and end on the cover (so it is contained entirely) and those that end on an edge (button opening, etc.) The later will tend to grow over time.
In general, I would be curious of HOW those crack happened. Can you imagine a force that created that? Internal PCBs, sensors, mechanics are rated for some acceleration, but not beyond... And if the sensor moved by any chance, you will get a continuous softness on your photos.