Just make sure the buyers are putting official cancellation requests in, where all you do is then accept or decline the cancellation. If that is the case, you shouldn't have been penalised. If however, you are the one initiating the cancellation process (even if the buyer has messaged you directly and requested it), it counts as a defect against you. Buyers are getting clever with this, because they know if they start initiating too many cancellations, ebay in theory, will look into it.