If someone buys something and then sells it on for a profit, they've profited for a reason. For one, identifying the arbitrage opportunity (profit potential) takes knowledge (and knowledge isn't cheap, takes time, experience and research)
If the seller makes a bad call they could make a loss (so there's risk). The reseller also has to put cash into the transactions to make it work and cash has a time value associated with it.
The reseller may market the item better than the person they bought it from or may be prepared to wait longer for a buyer. It's all really quite reasonable.
I sold a bike a while back knowing that the guy who was buying it from me was going to try and sell it for a profit - no worries, his risk, my convenience.
We all makes mistakes selling stuff, it's how you learn.
*Everything above assumes there isn't something illegal or highly unethical going on - illegality and ethics also have a price