I don't think an item of 100 diamonds or even more will make the market unstable. This item is not supposed to be used on simple pokemons, it's supposed to be used on very rare ocasions with very rare pokemons, like mythic or shinies. So you buy it very rarely because you know there won't be another chance to get the same pokemon, it won't even affect the price of these very rare pokemons, because they are very rare.
who da hell has ever caught 2 red gyarados? oh this one got intimidate, no problem, i'm gonna catch another red gyarados, c'mon stop being lazy and bring good arguments.
my opinion, thanks