If you think you want to buy a serger this week, take a brief walk down the aisle with me....
Buying a new Serger - the important things to consider are - is this model locked into a specific dealer in your local area? Dealer models - particularly Babylock and Viking brands lock you into using a dealer for everything or it invalidates your warranty, and by the time your warranty is over, you are so used to doing everything with that dealer, it sort of builds in the "upgrade path" on their treadmill.
How available are the parts for the model you are choosing?
Do they have an active User Group anywhere you can join for help?
How many YouTube or other video tutorials can you find that are based on the specific machine you are considering?.
There are 2 basic types of domestic sergers: computerized models and mechanical models.
The mechanical type of serger can be self-maintained, self-repaired in almost all cases, self-cleaned and parts are usually available locally and via many retailers on the internet. This winds up saving you a lot of money in the long run.
Computerized models offer self-threading which is patented in the Babylock and Juki products, which is great if you think that is worth all the extra money to keep the self-threading mechanism working, but beyond that, very little difference between these and any mechanical model. Another issue with any computerized models that offer 2-in1 type machines where it is a combo of a regular serger and a coverstitch machine - these sound like a great solution until something breaks. The repairs are very expensive. Despite what all the Babylock lovers will tell you - they do break, and the do require periodic "factory-authorized" maintenance. The bottom-line is if you buy an expensive machine - expect to keep paying for repairs and maintenance over the life of the machine - forever, and be prepared for the delaer to constantly try to upsell you into the next up model that has "more features, more bells and whistles and of course will give you some trade-in allowance for your older one if you bought if from them. It is a treadmill once you get on, you will probably never get off.
Mechanical sergers have less hassles but you have to learn what to do and what not to do - to have a happy experience with one. A common complaint on serger groups is that they are poorly made, a piece of junk etc. They are NOT poorly made pieces of junk - it is usually a matter of the User not taking the time to learn the use of the machine properly and they cause the machine to break by using it incorrectly. If you buy a mechanical serger learn how to use it properly and keep it cleaned and oiled and it will really last you for many many years beyond the one year warranty they all offer now.
Buying a used serger should involve more than base price and if it works. Are parts still being made for it, are they more expensive than the purchase price? Is there anywhere local that services that model? Are there any YouTube videos for how to do procedures with that model? Is there an active User Support Group for that Brand and Model?