Bryan,
My 2 cents…
Seller says the only part of the interior that’s been replaced is the package tray. This would suggest it was likely water damaged/warped at some point. Luckily it appears the original speaker grills made the transition to the new tray.
Additional possible clues to support this theory:
• judging by the seams under rear window not lining up with deck lid gaps, the vinyl top has likely been replaced. Which usually means rust remediation around the back glass has been performed
• minor rust issues in the trunk support the leaky back glass theory
Which means the suggestion by Keith and Alex makes sense to see how the frame, chassis and other things underneath look. While sleuthing around under the car (ideally the seller can arrange a lift for such viewing) check for those all too important numbers of which Dennis speaks.
Good luck and keep us posted.