Before buying any sewing machine it is helpful if you take into account what your sewing needs are. A model 15-91 is a true workhorse and it can handle anything from linen to heavy upholstery, drapes and even boat sails. It is one very reliable machine and it is also very easy to service yourself. It is also convertible to a treadle, which is a big plus to many folks dedicated to using machines without tails (meaning no electricity cord.) The price of $50 sounds pretty reasonable, especially if it has any accessories and an owner's manual. Most of the 15-'s I have here cost more than $50 -- yes, I do have a few of them.
Terry -- in Northern Delaware (my wife's favorite quilting enabler)