I guess you also have to take into account distance. Those chips are only worth $1000 in Vegas, they aren't like cash. I've definitely seen $100 in chips for $90 on Craig's List before, just because the seller doesn't plan on going to Vegas in the near future. With the pandemic, people are even less likely to travel, so I could see selling at a discount just to unload them.
At the same time, when the chips get up into the $1000+ range, uninformed buyers run the risk of getting caught up in illegal activities. Think of the Bellagio Biker Bandit turning to selling the stolen chips online, or even some kind of money laundering scheme. Frankly, the act of simply walking up to the cashier with 2 $1000 chips, trying to cash them, and then walking out likely raises red flags with the casino, who is also trying to prevent theft, money laundering, etc. Plus the normal risks of transactions with online randos still exist. I would personally be leery to buy $1000 casino chips from someone online, so it doesn't seem unreasonable to me for someone with $2000 in chips to be selling them at half of face value or less.