It's not the same way you are farming stackable items to sell to NPC or AH. Other items that you can NPC are not distributed in greater quantities the more AFK people you have in your alliance. You won't get 18 times as many dark rings if you are in a full alliance of leechers. If you did, that would have been nerfed so hard that even typing the words "dark ring" in the game would automatically disconnect you. Furthermore, dark rings, in this example, are farmed specifically to make gil, except the two best ones you find, which you'll wear.
As for AH sales, that is even more different. There is this thing you might have heard of called supply and demand, and there's a lot of it going on at the AH. It leads to products that there is an overflood of not being valued as highly as items that are rare. I am sure you can agree that cruor isn't rare, and I'm pretty sure you can agree that it is not in high demand either, so why should you get a lot of money from it?
You're talking about buying other people's time and basic economy. However, whose time are you buying when it took them 0 time to get the gil? If someone is going to buy your time, you have to give up your time. you can't both sell your time to someone else, and at the same time spend that time on yourself. That's the same as having your cake and eating it too.
Probably a long time, as you can't get dom notes afk. you have to be there and constantly talk to the npc. Unless there is another popular bot I don't know of.
If you want a gil source that is extremely unlikely to be nerfed, go for methods that don't rely on NPC selling.