Why is Re-selling Products exploitation?

Let's say a blacksmith creates a set of armor for modern armored combat tournaments. He sells it for about 1 500$ dollars. I then choose to re-sell this set of armor to another person for 1 800$. How is me getting that 300$ dollars an exploitation? Let's say I continue this process until I get enough money off from this re-selling process in order to to buy two such sets of armor at a time, which only allows me to continue buying and selling those sets of armor. How is this sort of voluntary exchange an exploitation? The same question goes for paying wages per hour for producing stuff I intend to re sell