sorry i havent been in here in a while... i am undergoing comp issues... sorry...
but.... to the topic... XD.....
selling certain items during certain times is the key....
some items can depreciate(go down in value) or do the opposite...
when something comes out... for ex: a monthly collectible..depending if its a good one, the value will go down during the time it is availible to everyone..... but once the the item leaves the cash shop.... at that point the demand of the item could increase and with the supply being limited the price can increase...
this game is really based on a supply n demand type of economy... the lower the supply and the higher the demand will make the item very high in cost.... keep your eye on things that u can afford... that someday will be worth more with time to come...