Is it a good idea to use second-hand track?

Personally, I would never use second hand track as you don't really know what you are getting.

Over time track will wear and tarnish and may even rust (for very old track made of steel) and its conductivity may become diminished even after giving it a good clean.

Visually some sleepers may be damaged or some paint may have been used on part of the track.

Even cleaning it does not really make sense, as it takes maybe 10 to 15 minutes to thoroughly clean the track and even then you are not sure some conductivity will not be lost. If you value your time, it is cheaper to buy new track.

Another potential issue is old track can be twisted, bent or otherwise not correctly set any more. If the track was removed from another layout, some ballast may be stuck to it which may also cause running issues.

So, I always replace old track with new to make for a smoother running track.