1. know the prices of a wide variety of items (this will allow you to know when you find a good deal to buy it)
2. know how to bargain
3. be friendly
4. snoop around the fm and know your competition's prices so that you can give your customers a better deal
Keep these 4 key steps in mind and you should have no problem! And of course, don't forget what I like to call the 'Merchant's Motto'! Which is Buy low, Sell high!