The game already has a temporary buyback option if you accidentally sell the wrong item to a vendor. (The only issue there is that you have to realize that you did so pretty quickly, since it only lasts until you leave the zone.)
It certainly seems like these sort of items bought with other currencies or trades are just as worthy of such a temporary undo option on purchase, and for the same reasons.
Another minor thing that might help at least sometimes is a slight adjustment to the warning message:
As people have mentioned, that warning that you can't equip an item doesn't help if you're buying it in advance of reaching the right level and therefore expect to not be able to equip it. To help with that, perhaps there need to be two differently worded warnings: one that you get if your current job can never equip that item, and a different warning if it's for your current job, but it's only due to your level that you cannot equip it.
Then you'd just need to be on the job you're buying for and a wrong selection would say so in that warning. If you were expecting "You'll need to reach level X before you can equip this. Proceed with the purchase now?" and instead saw "Your current job cannot equip this item. Do you really want to purchase it?", that might raise a red flag when you need it to.