first your going to need to have a decent amount of money to start your merchant life. I would suggest having at least 100,000 credits to begin. After you have a good amount of credits, head to the closest space station.

Once you have reached the space station put everything you have in your exosuit into your ship(or as much as you ship will allow). You will notice that there are ships going in and out of the station. I have made lots of money from Dynamic Resonator
In order fo you to make profit off selling them you need to buy them while the price is low.
As you can see when i buy it the price is lower than the galactic average and is shown in green to indicate that i would save money buying it. You can buy them from the larger ships i have noticed they seem to always have some. After you have filled you inventory you will need to sell them. The smaller ships are usually the one you want to sell to as they are willing to pay a high price.

You can tell if its is in high demand by looking for the golden stars that appear by certain items, there are almost always golden stars by the Dynamic Resonator when selling to smaller ships, there are some cases where that is not the case, but for the most part I have had luck with the smaller ships.

After repeating the process I was able to gain lots of credits. Watch your self be flooded with LOADS OF MONEY!!