Depends on how much money you have really. The easiest way in my opinion is flipping. Low return but very low risk. Basic items you can do that with would be runes for example. The more of something you can buy, the better it is. Buy say fire runes 4 gp each overnight and sell for 6 gp each the night after. Just an example.
If you want bigger returns, you'll have to do expensive items. Just pick out something fairly stable and buy a little below then sell a little above market.
If you want to play the long con, stock up on items you know will increase in price in the future then dump them at the right time. A good example of this would have been the master's wand for raids. They almost doubled in price because they were required to create the kodai wand.
Not the biggest mercher myself but that's the advice I can give you.
