OK, fatquarters did share with me the source of the ad.
And, I have to say, my normally distrustful, watchful, judging self ...wasn't totally put off by the ad. Surprising. It was a combination of the site/location/wording/pictures, etc.
If that ad was nearer to me, I would definitely have followed up on it. I actually would have traveled a couple of hours to check it out.
So that said, the original questions was what should fq should ask about it - if the seller ever calls her back.
As the closest I've ever been to one was at a show (where I drooled), I'll let others address specific 820 issues.
Here is a list of things that come to mind:
Looking at the pictures, I don't see a walking foot or 1/4" foot so you'd have to buy that on your own. That's around $200.
I'd want to know where the machine was bought, last time it was serviced, and ALL the paperwork from the original purchase (if they still had, and they should).
And, if you don't already know, call your local Bernina dealer to see what you'd be covered for if you brought in a machine you didn't buy from them. I just say that because I've been standing in a store when the owner told some people certain things wouldn't be covered if they bought the machine elsewhere. You need to understand what's included the warranty BEFORE you buy it. Somethings are lifetime covered by BERNINA, but that could be just the parts and you'd have to pay the labor.
Don't go alone. Since it's a local pickup and I'm sure you'll be paying cash (or via money order), you'll really have no recourse if something goes wrong. So you need to be sure and safe.
Bring some thread and fabric scraps so you can play - just in case they have none. You never know.
I hope he calls you back so you can get to the next step.
I'll be looking forward to how this pans out.