This may be a long process, but the profits are worth it. Look around for cheap deals. Find things which are GREATLY under priced (i.e.: Eluniums for 30k) and buy and re-sell them. Sell them cheap too, but priced enough so you still make a nice profit.

A good way to find deals is to listen to the merchants spamming Prontera with the things they want to sell. If they’re spamming what they have to offer, it generally means they need the zeny fast and will price their items cheaper to sell fast. The same goes for merchants buy-spamming. If you have an item they might need, the chances are they need the item fast, and will pay a good sum for it.

But one key note is, if you are uncertain of the true value of an item, DONT BUY IT. If you've paid too much you'll be kicking yourself for it for the next month, but if you don’t, you have lost nothing.