International legal precedents of this century do not allow an art vendor to guarantee his sales. Simply put, if you didn’t see the artist paint, draw or construct a work, you cannot guarantee it. An opinion given by the vendor at the time of sale must be one of experience and study in that field.
Buyers can seek pro or contra opinions and they will always exist, however, those opinions must be obtained by the buyer before the purchase to enable an informed and independent evaluation. Refunds arising from post-purchase opinion will not be entertained lightly. However, with that being said, if a picture purchased from me is proven to both parties to be other than as described and evaluated, I will be happy to refund the payment for that picture, without prejudice.