I'm not in the RE industry but I'd think there are a few ways where this could have been unethical, especially if the Seller's Agent didn't get the best prices/create a bidding war that should have occurred, represented both the buyer and seller without letting both parties know, or knew that the buyer was an agent and didn't tell the sellers (prob more of a grey area).
Not enough details from the OP (of this topic) to know if any of these things happened. Agree it will probably be a flip, which calls into question the selling agent's assessment of the true property value. But if buyer paid the highest price, the selling process was above board, and the price makes sense versus other comps, then it sounds to me like the agent's assessment was fine. RE agents speculate on property price appreciation all the time, so that could easily be what the buyer is doing here.
Not sure why everyone is so angry, could have been a good discussion if people had been a bit more civil, imo.