Strange that she would not let you look at the bobbin case. I think anytime a seller doesn't let you thoroughly check something out, that's a red flag.
It sounds like the top thread may not have been properly flossed into the tension disks. If so, then that would cause the nesting of the top thread under the sample and would also lead to bobbin case clunking as the excess thread could get caught up when the needle goes down.
I wonder if perhaps the seller just doesn't know enough about the machine and was thinking there was a problem, when it may be easily fixed?
If you have a chance to go to some shows you can test out an HQ Sweet 16 that is properly set up and working to see if you'd like it. That way you'll know what a properly working one feels like if you run across another used one. Be sure and take lots of sandwiches made of the materials you like to use, and also take some of your own threads.
The dealers love it when folks who know how to FMQ ask to play with their sit downs at the shows. I spent several years going to the larger shows near me and asking dealers could I play with their sit downs--- every single one was very accommodating, even when I brought my own materials and threads and asked them could we change out to see if my oddball thick/thin sandwiches and multiple different kinds of threads worked well in their machines.
Good luck with your search. The right machine will turn up.
Rob