S.!.
I just recently purchased mine off of craigslist and LOVE it so far. I went to a sports shop and talked with an associate first. I got all the details of what to look for from a sales rep and then went home and found the machine I wanted on craigslist. Of course they were trying to sell me a new one and I didn't tell them I was just using this info to go and buy used, but it still gave me a lot of info. I was able to decide what brand and what model I wanted by talking with a sales person at the sports store.
BTW.. I bought a Horizon T500 and so far it has been great. Don't forget once you decide on a machine to research the reviews as well.
Good luck!