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.