I'm sorry I didn't see your post earlier. I have the Teeter Hang Ups Inversion Table, it's more expensive that all the others but it has been put through many tests that are very important where other tables haven't stood the same testing and broke.
I wouldn't purchase a used one due to the fact you'll be hanging upside down and it could be very dangerous if you bought one that had something broken on it you might not be aware of.
If you buy one you'll never be the same again. It does so much more than help with backaches.