Save the seals for promotions and later on Squad/Command missions and then buy Ventures for your retainers.

As a healer you will get enough gear through the story and dungeons really, at level 1 to 49 you don't need to worry about having the best for your level as you wont be tanking (hopefully) taking damage (defense on gear) so the extra stats like Mind isn't going to be very noticeable. At level 50 you will get two sets of gear, then at 60 and finally at 70. I've leveled White Mage to 70 on eight characters and had managed to get from level 15 to 50 with just Novice Hall starter gear and nobody notice because it doesn't affect your healing much at all as your only getting +1 per so many levels. Not saying you should do that what i do, but it is possible and you will get lots of healer gear from dungeons anyway.