I would consider any of the "odd ball" chips to be "one of a kind" and not expect to find published rarity or value information on them. Generally, you won't want to pay the cost of a test of the core to determine the material. It will usually be more than the value of the item you're considering. Treat these as R-10 items, rare but not necessarily valuable.

Exceptions would be if the item offered is described as having a precious-metal core, like the Harrahs platinum presentation piece that sold in the Platinum Auction a few years ago. These pieces will generally be valued in the thousands of dollars and you will want to investigate thoroughly before investing in one of them. Frequently, these will be offered at auction and there will be lots of information presented. You will also have the reputation of the auctioneer to consider.

If a special-metal-core item is offered to you by an individual, then yes, you are going to depend on the reputation and integrity of the owner of the item. I would want to deal with a CC+GTCC club member and hopefully one who has been involved in the hobby for lots of years. As a buyer, you should have considerable experience in the hobby and know the right questions to ask before adding such an item to your collection. The value of a plated core item is mainly in its history and uniqueness, not in its intrinsic value. Remember there are only a few collectors of these. An auction may be the only way to dispose of it when you decide to do so. If you have decades to wait, the hobby may be entirely different when you decide to sell; your item may be worth hundreds of dollars, or there may be no one interested any more.