>>1187228Drivetrain, brake pads and tires wear out with use.
You can check chain stretch by putting the chain on the big chainring on the front and trying to lift a link - it shouldn't lift much up but if it does, the chain is too stretched and needs to be replaced. Chains cost $10-20, not a big deal to replace. A stretched chain also wears out the rear cassette, if the cassette is too worn it can cause the chain to skip in some gears. Cost of cassette depends on the type and number of gears, but usually $20-40. Front chainrings are more expensive, but those usually don't need replacing.
Brake pad life is easy to check if the bike uses rim brakes - they should have several millimeters of the pad material. Brake pads are very cheap to replace too.
Tire wear is also easy to see - if the tire has a visible tread pattern, it shouldn't be too worn and it's also a good sign if the sidewalls don't have visible cracks in them, but an old tire with cracks in the sidewalls might be just usable as well.
If it's not a super expensive/special bike you're buying with a high end groupset individual parts aren't expensive, but if you do need to replace the cassette, chain, brake pads, tires and maybe the gear/brake cables it can easily turn a $100 used bike into a $200 bike after all the maintenance has been performed.
