If you already own a bike you can surely think of several things that need tweaking, fixing, or tuning. I have some shifting issues between the 7th and 18th cogs when in the large chainring. But I am aware of them, and of how to deal with them so they do not impact my ride.

A used bike will have someone else's list of grievances. If you are handy with a wrench this is ok. If not, buy new.