There are some brokers who have a contract with resorts to transfer weeks to member accounts. This is usually with SA weeks, since having a Stateside Representative to handle member deposits is convenient. They do occasionally market weeks to non-owners in order to use up extra bulk banked weeks and also as a marketing strategy to encourage those people to purchase ownership at the resorts.
Since this has been set up with the participation of the resorts and RCI, this is considered OK.
The issue here is someone selling weeks on an individual basis. RCI frowns on this, since buyers can be mis-led about the trade value of the weeks, and then blame RCI if they can't get the exchanges they want.