I never see used sergers in our area, but know you can get a new one for around $300. I bought my Baby Lock in the 80's. It has four threads, although I can use just three. It has a differential feed and I have made several prairie skirts and innumerable pillow cases. I take it to get it cleaned occasionally and the expert who works on it says they haven't changed appreciably since I bought it. It does not have air threading, but once you know where the thread goes, you won't have any trouble., so I probably wouldn't pay extra for that feature. Besides, you probably won't need to thread it more than once a year.
Good luck, whatever you decide. froggyintexas