If you are buying platinum on eBay, you should be aware of a counterfeit coin (medal/token/medallion) that is occasionally seen on eBay.  You can see eBay auction 190197054377 or 190206790078 for an example (eBayer 'marketrose' is currently selling these as being real, even after being reported to eBay at least 3 times).  The obverse is nearly indistinguishable from a 2003 1 ounce BU United States platinum Eagle coin.  The reverse is similar to a 1 ounce proof platinum Eagle, but with a slightly different design (and "United States of America" and "In God We Trust", and ".999 PLATINUM").  This counterfeit coin is made of one ounce of a base metal (worth perhaps $1 or so), but probably plated with platinum (perhaps worth a few dollars).


 

To someone not very familiar with the real United States platinum eagles, it appears to be a real 1 ounce platinum coin, worth $1,000s.  To someone that is familiar with the platinum eagles, they will know it isn't a real one, but they will likely believe that it is real platinum, also worth $1,000s.

There is no doubt that this is considered a counterfeit coin, because it [1] has one side that is exactly the same as the original United States coin, and [2] the sales on eBay have gone for nearly the same price of a United States platinum eagle.  In other words, since this looks a lot like a real United States coin, people assume that it is (whereas if it was clearly a privately minted token, they would be much more skeptical and much more likely to question the real metal content).

However, presumably in at attempt to help prevent the makers of the coin from going to jail, there is a mark "5 mil" that indicates that (in conjunction with the ".999 PLATINUM" found elsewhere on the coin) is layered in platinum, rather than made of platinum.

It is apparently from a "Classic Coin Series" from the National Collector's Mint (which had a court order stopping sales of a similarly misleading medal).

If you see one of these coins being auctioned, I would strongly encourage you to report the auction to eBay as a counterfeit coin, using the "Report this item" link.  Remember that while some people sell these without knowing that they are not real platinum, there are fraudsters that intentionally sell them and do things like [1] show a picture of most of the coin, omitting the part with the "5 mil", or [2] claiming that they do not know much about the coin (for example, "I bought it at an estate sale, and don't know much about it"), and often [3] have the auction private, preventing anyone from contacting the bidders to alert them.