with how heaters can fail it's one piece of equipment that I wouldn't trust unless new (and even certain brands have been recalled, like the stealth!) The only used glass heater I'd even consider would be an eheim, but I had an old green one of those finally fail on me. New heaters have a guarantee if you get them at a big pet store too (and possibly from the manufacturer) just my 2 cents. Seen too many things fry or have heaters quit to take chances. Also get a temp controller as an insurance policy. Jo Jo also has good prices on new eheims.
I know, you are just looking for a used heater, LOL.