If I sold a camera to someone an at the time of sale it was in good condition but 33 days later stopped working, do I owe a partial refund?

This type of sale of a used item is understood to be in "as-is" condition, and the buyer takes his chances with regard to the quality and condition of the item, and with respect to problems that he may encounter with the item in the future.  If you did not give the buyer any express warranty or guaranty with respect to the camera, either orally or in writing, or make any specific promises about the manner in which it would function in the future (as opposed to the fact that it was in working condition at the time of the sale), then you do not have any obligation to refund the purchase price or any portion of it.