I require a deposit of $100 to reserve a kitten for a buyer.  I allow a cooling off period of two weeks and will refund the deposit during this time.  After this the deposit becomes non-refundable.  After the kitten has been picked up I allow a four-weeks "no questions asked" return period in case the kitten does not fit into your circumstances.  After that period I cannot guarantee a full refund but will always take the kitten/cat back for re-homing.