I have noticed this issue with out of date Reverb listings showing up well after the listing ended too.

I can cite an example: Right now you can look up the Pittsburgh Modular Lifeforms Touch Controller and it shows 1 listing on Reverb:
https://modulargrid.com/e/pittsburgh-modular-lifeforms-touch-controller

However, that Reverb listing sold in early May, almost 4 months ago.