I'd argue that a player consents to any actions that are within the bounds of the game, including the possibility of being deceived and exploited. Indeed, the game itself has had plenty of examples of this that doesn't have anything to do with other CMDRs, and I regularly have my CMDR engage in deception to accomplish his goals (I used to fill trade and exploration ships with undersized railguns to discourage interdiction, and still give cheeky names to my CMDR's unEngineered vessels to convince people that they are too fast to catch) and I will certainly have my CMDR bluff or outright lie through his teeth if that serves his purposes.
It's probable that these slaver players didn't have the best intentions toward their fellow players, and that is unfortunate, but unless they were violating the rules of the game it's also irrelevant. Indifference, or even antipathy, do not constitute harassment.