I'm new to serging but from what I've seen and read, most of the problems with sergers are operator error such as using the wrong type of thread, threading incorrectly (wrong threading path, threading in the wrong order), and a misunderstanding of thread tension. Many owners express frustration with switching from overlock to rolled hem and back, which on some machines is a time-consuming and tedious process.
I believe sergers take standard needles and new blades are easily found. I've often wondered if blades can be sharpened. I have a scissors sharpening stone and wonder if I could do it myself.
My buying advice would be similar to a standard sewing machine purchase: know what kind of stitching you need prior to selecting a machine because not all sergers make the same set of stitches. Look for loopers that are intact and make sure the presser foot is in good shape, as these things are expensive to replace and can cost more than the price of the used serger itself.