I find it hard to believe that so many people are poor in this game. One thing I like about SRO than FFXI was the ease of getting money. I could go into Jangan, and easily make 100k from buying low and selling high.
Some people say to sell armors that are not +1 or Blue/SOS/etc to the merchants and then stall everything else, but I find that not entirely true. As long as I don't get booted from the game, I can stall at least 100k a night. I have had over 500k since level 14, and I am an INT player that doesn't rest while playing (lots of potions). I also have +STR on nearly all of my equipment.
They key is to plan ahead; if you see something that you will need reasonably soon for a decent price, buy it and put it in storage. It is better than reaching a level where you have to spend hours hunting for a decent weapon or armor and end up paying too much for it! Also, you could put your elixir drops to good use by making your next level weapon really nice. I did this with a level 21 +3 STR spear that I purchased at level 17.
+ Quests will knock two birds with one stone: they give you XP per kill and they will give you XP when the quest is done. The money isn't bad either. It is win/win!
+ Everyone wants the best equipment for their level, so even the regular armor will sell for at least double the store price. Make sure your stall is always full!
+ Selling Elements will make you rich, but for that reason buying elements will make you poor
+ One star trade runs will net you AT MOST 60k with a camel, not including expenses. I do them for fun, not for money. If you start trading right at level 20, you run the risk of some powerful mobs attacking your pitifully slow camel/horse, or some asshat thief who thinks spawning his NPC's on you is great fun (seriously, a one star trade is almost worthless to a high level thief, why thieve it?). Angst ridden kids love making your one star trading a nightmare in this game.
+ Stall, stall, and stall some more. No danger of thieves and more money. Leave the game up while you sleep with your stall open.