First of all I apologize if this is in the wrong section, not sure whether it is general or quilting talk.

I have only one sewing machine. I would really like to get at least a backup. I know a lot of you have many machines you have acquired used. How do you fiqure out which one is a good buy and most important will work?

Emma,
Lots of times I have researched the older machines so that I know what I'm looking for. When I decided that I wanted a Pfaff that had the IDT(full time built in walking foot), I researched the different Pfaff models that came with that feature.
I also ask questions of others as to what they like and don't like about a particular model of machine.
To be sure it works, I test drive it if possible with my fabrics and threads.
www.patternreview.com is a good place to start. You will find a section that has reviews of all sorts of sewing machines.

Hi Emma, Yes, I felt the need to have a back-up I got a featherweight that I'm loving!! It only does straight stitching but is just what I want to have downstairs or to take to a class. I would try the machine shop here and ask Billy or Charlee or Miz Johnny. There are many answers to this question.
Mine is to get something that works well and fits your budget.
PS. I love the older machines for a back up b/c they are well made and don't have lots of things that will go wrong.

I would decide which features I must have. On a second or backup machine, you may want to go for those features that your current machine is missing. Then I would look for at the different brands to see which machines meet my needs in my budget. Then I would read a lot of reviews on those machines.

I believe in talking to dealers. While I am no good at "dealing" in general, I have become a relatively good negotiator when it comes to sewing machines. Ask people in your area where to go. Who is good (sales and maintenance). Who will negotiate.....and have fun.

I'm thinking I want a no frills, heavy duty machine. My main concern is how do I figure out whether there is still some life in the used machine. For instance, my daughter left her machine in storage and the inside rusted, totally worthless. In other instances I would imagine the seller really doesn't know what kind of condition it's in.

Many times a machine will "speak" to me. There is not much that can go wrong with the really old machines. If you turn the wheel and the needle goes up and down, it works. Just a matter of cleaning them up and learning to use them. I have a top of the line Janome, way too much money, my grandmothers Singer treadle 1918, a Featherweight 1938 and over the weekend I bought a Hus.Viking. It is pathetic, but will be beautiful once she is cleaned. So it is up to you. I say a Singer 301a and fell madly in love with it, a black one, so that is my next conquest.

when i look for a used machine i would ask if i may try it out and if i don't like how it sews i will keep looking until i find one

I have Singers 301 and 221 Featherweights too. Very nice machines that sew well and light enough to take to classes or quilting bee.

LOVE my 301!

You're right that it is buyer beware - that some machines could have hidden problems like rust inside.
My tech also warned me that some heavy duty machines can look great on the outside if they were used on a frame and be totally worn out inside.
And buying second hand privately carries no warranty.
For that reason alone, I won't pay more than $50 for a used machine privately.
When I look for a workhorse machine, I go to my dealer. He's the one who fixes them too.
I just added a Janome Horizon 7700 to my "stable" and Love, love, love it.