Remember that minor differences can result in big gains in profits. Know your numbers. Most people mark up their products with too little mark up. Always remember that you have to get at least a four to one mark up or better from your cost to what you sell for. Sure, some products have low mark ups because they re looking for long term customers.
Some people lose money on the first sale just to get new customers.
They know, of course, that they can lose money up front to get customers. My advice is make your profit in the first sale when you begin, for two reasons. One, losing money on the first sale works well if you have enough money to lose, and you know for certain that people will buy more from you. Secondly, you know how much you can afford to spend to get a new customer. Until you know these things play it safe.