The secret to it is mushrooms, giant mushrooms can be grown with the free bonemeal we get and they drop anywhere from 2-25 mushrooms, these can then be sold at the admin shop for around 4k a stack which only takes about 5 mins to get (use a diamond axe, it will repay itself very quickly)
To get these mushrooms you will have to buy them off a freeman, there is at least one chest shop which sells emNow to make a room for growing these just build the biggest room you can in your plot, it has to be at least 7x7x8, if you want you can buy some mycelium off of a freeman and not worry about the light levels but mushrooms have to be under 11 to stay planted if you use normal blocks, i would use cobblestone as dirt is expensive
then whilst you are chopping your mushrooms down you may as well set up a farm, i recommend a multilayer farm to maximise space and to grow sugarcane, this seems to be the most efficient moneymaker
Also as many of you know, i quite like pvp and i am good enough at it to kill people and have made lots of money doing this, now, you could fight honourably and stand out in the open and give everyone a fair chance, but that would be stupid
ambushing people makes life so much easier, just use iron armour and a lvl 1 enchanted diamond sword and you will soon be making people rage at all their lost diamonds that you can sell for a nice profit.
I hope this helps people out in making money