I am a mechanic for such a company, and a poorly maintained bikes are bad for business and even worse for the mechanic. A rental bike must operate smoothly or the rental company will end up refunding and fighting credit card companies. I won't allow a bike to be sold unless it is in good working condition and does not need any new parts when it is sold.
Test ride it like you would any bike purchase. Ask about return policies, but many used bike sales are final and non-returnable.