Hi, I own 2 necchi sewing machines my self. The first necchi I owned lasted for around 15 years or more, and I traded it in on the one I have now, a necchi royal, I had it for around 15 years or more and it's still in great shape and works great, the other necchi I have is a electronic sewing machine and my dh loves quilting and sewing on it..
since this machine belongs to a individual, I suggest asking them to let you sew on the machine and see how well it makes a straight stitch and zig zag, also does it come with a instruction manual? has it been taken care of? are all the extra feet with it ? has it been serviced regularly?
if you get to sew on it, try out all the stitches to see how the machine handles, that way you know if the decorative cam gear is still in good condition, make sure the feed dog feeds the fabric smoothly.. but $30 isn't bad if it's a good machine..good luck with your decision...