The shop has a bug. The two accounts I have which weren't around for the Lavender event can exchange their seal stones, but my main account which was there for the event but didn't get to max her skills/slots has a blank spot where the stone exchange tab should be. It also has a blank space where the new life crystal exchange tab should be, so it isn't likely some stupid 'you were there, you should've tried harder with your brand new account' shenanigans.