Frankly, I can't believe people still go out game hunting and find something second hand at all. I stopped doing that about 3 years or more ago. It's all retail, gaming cons, or occasional eBay now for me. But I have a really good local store with a massive selection, so it don't really miss the hunt. I've become accustom to the high prices now, don't even care anymore.
If deals really dried out you could question why retrogame stores still exist, just saying live isn't free not to mention that allot of retrogame stores have enough cash for employees and rent for the store every month. You can't live from only trade ins it has to be way cheaper than that to keep the lights on.
as far as fleamarkets etc go. Maybe stuff isn't dried up but just the simple fact that more people are hunting and picking stuff up. In the past no one hunted for certain things however if there are multiple people even 2 to 4 people looking for the same stuff on a fleamarket the good things are gone. In the past maybe some people were the only ones looking for stuff on a flea market. Stuff is not plentifull there is just a certain amount.
thrift stores, maybe only a few people looked every week through games etc yet I'm pretty sure at least dozen of people pick through certain thrifstores daily. Not even resellers but people who do it for fun wich they did not in the past thanks to the retrogame hype or that some stuff have value.
You just need one or two people to buy everthing up if the prices are low, in the past the competition was pretty minimal only for the few people that actually cared about the hobby wich were pretty rare back than.
While fresh supply of retro stuff will be less and less when times goes on I'm pretty sure that if the competition is the same like now even if stuff is more plentifull like back in the day allot of folks will barely get anything and complain like they do right now.