It is easier to make money once you have a little to invest in making more. Stocks are good but have their good and bad days. My favorite way to make tu is to do item Quests.
I find that the Squishy Quest is the easiest to progress in since they are easily available. There is always a wide selection in NPC and User shops. My quest method is to purchase one of everything in stock in the NPC shop each time it restocks (ever 15min). This can be done while you are looking for items on the lower quest levels since you have a lot of time to turn in the requested item. Just hang out in the NPC shop and buy everything you can until you get what you need! As you amass your own stock of items the quest becomes easier since you have the items already! This becomes important at the higher levels when you have very little time to turn in the items.
The higher the level, the better the pay-out. The pay out for Item Quest games is always higher than NPC shop prices. Usershops are hit and miss - but usually it's okay to overspend in usershops just to get to higher levels for a better payout.
I believe the Quest game with the highest payout is the Weapon's Quest - but I think it is more expensive to have a stockpile of those items.
Hope that all makes sense. Best of luck making tu!