That's the common misconception.. Because how crafting is so mind numbingly boring, people overestimate the time to craft such a large amount of items.. In the span of about 3 hours, I could make 4 stacks of horn glue Which at the price I posted them, would net me over a million gil. That's not to say that it will make me 1 million gil in 4 hours.. You have to let it sit and wait for it to sell which can take a few days to a couple weeks.. So you expand this operation to 3 or 4 different itemss and maybe you can have enough workload.. It all depends on the player of course, personal preference plays a very large role in this.
With this method, you can take out the time to sell out of the equation because you don't need to babysit the items with price wars (nobody's going to undercut a hq horn glue at 3000 gil)
What you DO need, is spare retainers, because your main needs to be doing higher demand, big ticket items which is what I use my main for.
Masamune's economy... I'm not touching that with a 40 foot pole... and I do believe some of my customers may be RMT related, can't say for certain though, when I've felt lazy I've spent stupid amounts of money on mats just because I didn't wanna make them.
Usually, it's one person for a bout of 2-3 days then they're done.. And a new buyer will come along in a few days to a week and buy a few more stacks.
In the case of horn glues, the large stacks are 2 different people, with 1 person buying the majority.. he's probably farming chocobotail saws. But all the smaller stack sizes are different people.
Again, this method isn't for efficiency or perhaps even most money made, this is about making gil without having a 4 star crafter and without fighting the monopoly.
Reason I mentioned getting extra retainers for this isn't because of inventory management, it is because you need extra sell slots that you can sit on for days/weeks without worrying about it.. I couldn't do this on my main retainers, they have more important things to sell.