When a listing ends, most systems remove it. The part is gone, the listing has done its job, and keeping it around costs storage. That is a reasonable default for almost every kind of commerce, and it is the wrong one here.
The day you need it
A buyer opens a case eleven weeks after the sale. They say the part was described as tested and it was not. You know it was, because you tested it. What you need is the listing as it stood at the moment they bought it: the exact grade, the exact condition notes, the photographs, the timestamp.
If the listing was deleted when it ended, you are now arguing from memory against a written claim. That is not a position anyone wins from.
Where ninety comes from
The number is not ours. It is eBay's return and dispute window. Retention shorter than that leaves a gap exactly where the risk is; retention much longer than that is storing data for no stated reason, which is its own problem under data protection rules.
So ended listings stay queryable for ninety days, complete, and then a retention job removes them. The number is a policy, not a preference, and it is written down so that when someone eventually asks "why ninety?" the answer is not "it seemed about right".
The general rule
Any default that quietly destroys evidence deserves a second look. The cheapest moment to decide what you keep is before you need it.