If used, the PTO shaft is usually already cut down a bit. However, if there is 6" of engagement of the sliding (telescoping) shaft when it is hooked up to the tractor then it is good to go. Sometimes even with the Pat's system the PTO shaft is long enough to work safely.
Tractor Supply also has replacement PTO shafts but they don't come cheaply. That is why the extension (adapter) in my previous post is a good choice for used equipment with a PTO shaft that is slightly too short.