Currently, trying to purchase an item that the character can’t equip will give a pop-up saying “Unable to equip”.
This prevents players from buying exchange shop items for their other characters (medals should be transferable to begin with - I stopped buying goddess cubes because of the trash medals being stuck to one character) or any outfit that they simply wish to trade, sell or gift.
It is a good idea to warn the player they won’t be able to equip the costume, but you can make it a warning, not a restriction.
A yes/no pop up should appear, saying something like “Your character won’t be able to equip this costume because they don’t meet the class requirements. Are you sure you want to purchase it?”.
It could also appear in the market, since people might make the mistake of spending a fortune on the c3 costumes they can’t actually wear.