Hi Elisea,
As the other replies have indicated it really depends on your own personal sense of how comfortable you are either sleeping or having a guest sleep on a mattress that has been used by someone else. Some people wouldn’t consider it and some would be fine with it. It would also depend on how well you knew the person you were buying it from and of course any costs involved compared to buying a similar quality mattress new.
Post #2 here has more about buying a used mattress that may also be helpful although it was a mattress that had a zip cover so the inner materials could be inspected as well (which can often have hidden issues) which may not be the case with your mattress.
Phoenix