Your best bet if you haven't leveled a healer up is to max out items, convert them, and sell high tier materia. For a while you'll be able to make some money selling L20+ botany/mining stuff, but don't count on that lasting long. The economy in this game isn't even vaguely well-designed enough to generate consistent revenue, and as more and more people max out classes and train up their DoH/DoL classes that's only going to get worse.

I actually make far more money hiring myself out as a mercenary healer for the level 15-30 dungeons than I do through any other channel. The vast lack of healers pre-30 means that DPS tend to be happy to pay 2k-3k per dungeon run to skip the 1-3 hour queue, and healers are the big bottleneck at lower level right now.