Unless I know where a used welder is coming from or it has some sort of warr. on it from a reputable dealer I will pass. I have seen the way people treat these machines especially if not owning them and even then they get beat up. For the difference you can buy new. Unless you possibly get one from a friend and know the history. JMHO. I am sure many have done this with good results.
I, of course, like new stuff better. I just keep coming across posts for used ones and that got me thinking. There are a lot of things I wouldn't buy used, maybe a welder should be one of them.