No, there is no legitimate way to sell your unwanted coupons.
The sale or transfer of coupons is a violation of virtually all manufacturers’ coupon redemption policies. These policies are generally printed on the coupons or are available from the manufacturer upon request. Any sale or transfer voids the coupon.
People purchasing coupons have often been associated with organized criminal activities. They often purchase the coupons as one aspect of a scheme to defraud the coupon issuers/manufacturers, usually by seeking to redeem coupons without purchasing any products. Individuals selling coupons to such crime rings have been charged with and convicted of criminal violations.