redfernclan,, the question was about making racks or displays for coins,, NOT for selling the coins. From all I've ever heard,, selling a challenge coin is taboo.
When I was in the Army,, I knew about special units that had a Challenge coin. My unit didn't have any nor did any of the units we mostly dealt with. From that era, it was the special ops units that did have them. As such,, even though I'm a Vet, I was never gifted any real Challenge coins. But I do have a Challenge coin so to speak,, gifted to me by THE man. "The Gunny" himself,, had a coin made when he was promoting Glock. I was unaware of it. We met briefly several years ago,, and he made a comment about the shark tooth necklace I was wearing. I had a few shark teeth I had found & wrapped for a necklace. I got one out of my pocket,, and gave it to him say; "Here's one for you,, for good luck!" He was genuinely surprised & liked it! As I was about to leave,, he shook my hand & slipped me a Coin. I treasure that one. Only one I have ever been gifted.
But my son,, he has been gifted several by other LEO agencies & he has a rack for his coins. His Texas Ranger one, and his President Trump one are among his favorites.