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I'm still finding souls pretty profitable, just not as much as those first few days. Overnight I can collect 80-100. And I find there are two ways to sell them, not sure if you all do this already, but its very helpful.
If I wake up in the morning and plan on playing for a couple hours and need money fast, I set up shop right next to the event NPC with souls selling at about average price on Demian, 250k each. I also check the shops around to see if there is anyone selling for lower, and I either buy them if I think I can sell them for higher, or I go right under their price. At the average price, or just below it, within 15-20 minutes someone should come along and buy the whole stack, leaving you with a nice quick profit.
If I wake up in the morning and have a full day of work or school, I give the stacks of souls to my merchant alt (so I can keep collecting on my other char all day) and then I set up shop at the event NPC, but this time I crank the price up to 300k-350k. People won't buy stacks of these, since they know it is on the expensive side. But what they will do is buy small amounts when they have almost enough for a box. So throughout the day, they will slowly trickle out, selling one or two every few minutes. By the time I get home, they are all sold, and I have made 20-30% more profit than if I had sold at the regular price, and that can add up to 5-10m more.