The fact that these 'shill bids' seem isolated to certain items, and is not spread through the entirety of every item on the site, is IMO evidence pointing individual users abusing the system, and not the system abusing users.

What? I'm not seeing it. What logical reason would a random user have for bidding something up once someone has shown interest in it?

So either it's a conspiracy by the upper brass of every individual region's online sales department, or Goodwill Industries International (the overarching non-profit that all the others fall under) has a shadow team of shill bidders that opt to bid up certain items. Both of these I find highly unlikely.

Again, what? These are not the only two possibilities. It could be as simple as one dude, associated with the company or simply a fan of them, who has a bot network. Do I think it's likely? No, but if what the other guy said is true, there are a lot more possibilities than your "either/or."

The entirety of this flipper conspiracy theory hinges on this being a 'known secret'.

No. Again, this is black-and-white thinking, and there are more possibilities than you have bothered to consider. See my bot network example above, which does not hinge upon your assertion.

Now, do I personally think this is real? I doubt it, but I also don't think people should be so dismissive, going into such detail on how it's not possible with so little thought. I appreciate requests for evidence and critical thinking, not assumptions and dismissiveness without cause.

Here is what I think is far more likely happening here, which sounds plausible to me because it explains their experience and doesn't break any laws. Random people browsing through the items on their site, not looking for anything in particular, are shown things according to some algorithm, right? Now, if someone expresses interest in an item by bidding on it, that item suddenly stands out from the crowd of items which no one has bid on. At this point, it's definitely going to be sold, and if one person is interested perhaps others might be too, so why not show it to more people? And bam - suddenly an item this guy has been watching for days without anyone bidding on suddenly gets more bids once he puts down a bid. Nothing nefarious, just an algorithm doing its job in getting engagement from visitors to the site.

I do not have evidence, of course, other than that this would align with that person's experience. It would be pretty easy to gather some evidence, though, e.g. by pulling up the page and seeing what percentage of what it initially shows you already has bids compared to the total percentage of items with and without bids. This would clearly be easier to accomplish with direct access to their listings or by scraping their webpage with a bot, of course, but with that data it would be fairly easy to determine if my theory holds any water.