You can use seals to buy things like cordials to sell, furniture (the portraits are popular), and the non-binding stuff for the Heavensward Anima relic.
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I haven't touched crafting/gathering yet this expansion, but last expansion I made hundreds of millions of gil as an omnicrafter. Really that is where the most amount of gil is going to be made. Though it is an extreme time investment leveling everything up if you're starting from scratch. If you're just looking for some casual gil I'd at least recommend leveling your gatherers. Even a low level miner can make a million gil an hour selling materials that are required in all kinds of recipes. The omnicrafters rarely gather their own materials I've found instead favoring to buy from the MB in bulk and likewise craft in bulk so in that there is opportunity.