" Somebody else must have used her account."

If I was the seller, I'd be checking this out carefully.

WHO used her account?

Did she report it to the police?

Was it a relative?

I think paypal are able to use debt collection agencies, etc and impact on your credit rating and all the rest like if you don't pay bills.

That would be absolutely correct and for most people, enough of a worry that they would not want it to happen.

But I also know (from helping sort out someone else's major problems) that debt agencies will settle for a fraction of the total cost, in this case I would say about $3000.

But it is an interesting legal situation as they could possibly argue the caravan had been sold and the money was theirs.

I'm suspecting the buyer wasn't hacked, no one else used her account but she has had a change of heart or needs the money.