I usually write people and ask them if they have any additional of the same chip for sale. I usually get blown off 90% of the time, and my email is never returned. The other ones that return the email will usually let me know if they have more for sale, but never volunteer how many even when directly asked. They ask me how many I want to purchase instead.

When it comes to ethics, if someone is not disclosing rarity information then they are breaking the rules in my opinion. Any time someone is dealing in collectibles, perceived rarity is always one of the biggest determining factors of value. Especially casino chips that are decades old. It is implied by default that the existing supply is not changing or increasing. To not say otherwise, is unethical imo.
Now to be fair and look from the other side of things, there are buyers out there that don't care about value, supply, price, rarity, or any combination of the afformentioned. they just "need" to have something, I like to call them the "rich fools". To them the value and purchasing power of their dollar is not important when buying collectibles. They are in the minority, but can have an impact on market prices when demand is extremely low or "believed" to be extremely low.
But in modern times, there are few sellers that will ever put ethics above making money. We live in capitalist america afterall! (well, most of us). As a buyer the only safe defense is to get used to people lying, and investigate an item as much as you can with the information you have available on the internet (Thankfully that's a lot of information).