There is absolutely nothing wrong with the idea of buying a second hand vacuum cleaner. But there are certain critical questions for you to ask, before buying such a second hand vacuum cleaner.
Firstly, before buying a second hand vacuum cleaner, you need to pose the question as to how long it has been in use before. This will (indirectly) tell you how much useful life the machine still has ahead of it. And that will in turn tell you whether it is the best vacuum cleaner for the money you have.
Secondly, before buying a second hand vacuum cleaner, you need to pose the question as to why the seller is seeking to sell it off. This is a critical question, regardless of the specific type of vacuum cleaner you are looking at — that is, even if it is a very basic pet hair vacuum cleaner. If the vendor is selling it for a negative reason, he or she is unlikely to tell you – but then, you can read between the lines or listen to what is being unsaid after you pose the question.
Thirdly, before buying a second hand vacuum cleaner, you need to pose the question as to whether the machine has had mechanical problems before. This would be a pertinent question, even if you were looking to buy, say, a second-hand Fiat, Suzuki or Peugot car. Again, even if the vacuum cleaner under consideration has had such problems before, the seller is unlikely to tell you. But by watching his or her body language carefully, by listening to what is being unsaid and by reading between the lines, you can get the answer you need. This, at least, is the best approach to use: and even the best experts from, say, the University of Washington would probably aver to that.