I don't think it's a scam. Rather, (based on a quick bit of Googling) it looks like casinos track their high denoms, and the seller is hoping at least one buyer doesn't know that. It seems a couple folks have gotten screwed trying to cash in a lone high-value chip...
https://www.tripadvisor.com/ShowTopic-g45963-i10-k3628399-5_000_chip-Las_Vegas_Nevada.html
http://www.sportscapperisland.com/forum/poker-room/18518.php
I'm guessing this chip is legit, although cashing it in may prove to be damn near impossible. If you're a gambling man, you could try and get it into play and break it down for smaller denoms. But even then, with only one chip to put in play I'm guessing the croupier, dealer, pit boss, etc., would question the chip's source, and would not allow it to be put into play or exchanged.
When I say "scam", I'm referring to it's likely stolen, or otherwise obtained nefariously... The seller is trying to fence their ill-gotten goods.