i tend to keep a short list of things that tend to do better than others, and then check that list, then gather or make ONE of those things in bulk and then sell in portions.

I personally think there's a 'sweet spot' amount of gil people both have on hand and are willing to pay, but I'm not sure what it is, but im guessing its 2-10k. Seems to me a broke character can earn only so much gil reliably and those are the customers I want, because they can reliably crank out a few grand here and there and buy my reasonably priced stuff.

To say there is one particular item you can always sell to make X amount of gil is foolishness. Prices fluctuate by orders of magnitude and vary from server to server. Sell what sells, skip magic wonder do it alls. They don't exist. Even shards vary from week to week and element to element. Is it 14 gil or 145 gil? Who cares! Sell something that costs more, when it goes down, sell something more valuable.