I've had mixed results doing it online. Some good results some not so good. I've had excellent results by meeting folks online that were local, or through the newspaper, yard/garage sales, and some shops near me also will sell used stuff when a customer breaks a tank down.
It comes down to doing the research to find what you want, then looking for it used, and checking the condition of it. Just finding a piece of used gear that's cheap and buying it cause it might work is where lots of folks get into bad situations.
B