to US citizen's property nationalized by the Cuban government. The amount is up to about $6 billion now. Seems to me the issue is the chips were the property of the casino in Cuba, owned by US citizens. If you get a Cuban chip, you violate the embargo. Of course, the real driving force is ebay needs to protect itself from litigation (right or wrong).

We know many of the old chips were brougnt to the US by Cubans who fled the island and are now in the USA. You'd probably win a lawsuit alleging you violated the embargo, but ebay has millions of users and doesn't want to tie up lawyers in hundreds of lawsuits. Lots of other prohibited ebay items are prohibited for similar reasons; ivory, eagle feathers, products with famous makers' trademarks on them, etc. As a business decision, it makes sense for the auction operator to prohibit sellers of questionable items.

Sellers continue to list Cuban items by stating they are "pre embargo". Can't really be proven, but you can continue to sell if you are creative.