At the root, the problem is that Amazon's stock is not discoverable. Their search engine is terrible, I've had situations where I knew the exact listing I wanted and wanted to see if anyone else was listing it cheaper, I searched a few keywords and had the listing I knew about plus a sea of other listings, then I added more words from the listing's title and suddenly the listing no longer appeared in the search results. What in the actual fuck is going on there?
To be honest nowadays I just google search "[item] amazon" because the Amazon search is such utter trash. If it doesn't pop up in the best sellers list and you can't manage to filter down by "prime" and "ratings" it might as well not be there according to Amazon's search, it's just lost in a sea of junk listings. The terrible search is one of the major reasons why nobody buys low-rating items, and that's why people bought ratings in the first place, so really it all comes back to the terrible search.
For starters the results need to be weighted by number of items sold though that listing. The brand new listing from a brand new seller yesterday gets the same weight as the item that's been a #1 bestseller for years. But other stuff, like items going away when you try to narrow down to them... I don't even know what's going on there.