Do I have any options for canceling or rebooking or do I need to do it through that person?
Often, renting a reservation from a
DVC member is non-refundable and can't be modified, because of the way DVC works. You may know this already, but for others reading: DVC Members own an annual allocation of
points, and they use those points to reserve rooms. Unlike cash, points have an expiration date, after which they are useless. It is likely to be the case that the points used for your reservation now have a very limited "shelf life," and that combined with the overall low availability of DVC rooms means that the Member might not be able to re-book that room for some other dates, even if they wanted to.
This lack of flexibility is why a DVC Member will typically rent a reservation for a discount vs. what Disney charges. There is more risk in renting a reservation from a DVC Member vs. booking the room through Disney.
Unfortunately, there's not much we can do to help. You will have to contact the Member and see what, if anything, they are willing to do for you. Without a contract or some other agreement terms in an email, etc. you don't have much recourse. Again, for those reading, this is why folks on DIS usually recommend having a contract signed by both parties, so that expectations are clear.