While there is no reason to completely avoid buying a used instrument, you should still make sure that it is good quality. Ask to find out whether it stays tuned for a good amount of time, check to make sure the keys have even touch etc. Bear in mind that "new" doesn’t necessarily mean "better". If you are unfamiliar with the instrument, make sure you seek expert advise (and pay for it if necessary) because the cost of buying a poor quality instrument goes beyond the money that you spend on it.