pragma nexscript >= 0.2.0;

contract TradingPost(
    bytes32 tokenid,
    bytes20 seller,
    int rate,
    bytes20 provider,
    int fee,
) {
    /**
     * Cancel (Trade) Posting
     *
     * Sends all remaining assets back to the contract's owner (ie. the Seller).
     *
     * NOTE: For compliance purposes, both the Seller and the Provider have the
     *       ability to cancel a trade posting.
     *
     *       ASSETS CAN ONLY BE SENT TO THE SELLER.
     *
     * @param executor
     * @param signature
     */
    function cancelPosting(pubkey executor, sig signature) {
        /* Verify executor has provided a valid signature. */
        require(checkSig(signature, executor));

        /* Hash the executor's public key hash. */
        bytes20 pubKeyHash = hash160(executor);

        /* Verify the executor is an authorized contract participant. */
        require(pubKeyHash == seller || pubKeyHash == provider);

        /* Validate the 1st output is the Seller's `scriptPubKey`. */
        int lockingBytecodeLen = tx.outputs[0].lockingBytecode.length;
        bytes20 sellerBytecode = bytes20(tx.outputs[0].lockingBytecode.split(lockingBytecodeLen - 20)[1]);
        require(seller == sellerBytecode);

        /* Validate the 1st output is the Seller's asset. */
        require(tx.outputs[0].tokenGroupId == tokenid);

        /* Set (min/max) constants. */
        int MIN_OUTPUTS = 1;
        int MAX_OUTPUTS = 2;

        /* Validate the 2nd (change) output IS NOT the Seller's asset. */
        require(
            (tx.outputs.length == MIN_OUTPUTS) ||
            (tx.outputs.length == MAX_OUTPUTS && tx.outputs[1].tokenGroupId == 0x0)
        );
    }

    /**
     * Trade (Token) Asset
     *
     * Sends the buyer the requested amount of an asset for a rate set
     * by the contract's owner (ie. the Seller).
     */
    function tradeAsset() {
        /* Parse and verify Seller's bytecode receives the $NEXA payout. */
        int lockingBytecodeLen = tx.outputs[0].lockingBytecode.length;
        bytes20 payoutBytecode = bytes20(tx.outputs[0].lockingBytecode.split(lockingBytecodeLen - 20)[1]);
        require(payoutBytecode == seller);

        /* Set amount of $NEXA Seller paid to posting. */
        int sellerPayout = tx.outputs[0].value;

        /* Calculate and verify Buyer's asset payout amount. */
        int buyerPayout = sellerPayout / rate;
        require(tx.outputs[1].tokenAmount >= buyerPayout);

        /* Verify 2nd output is the Buyer's asset. */
        require(tx.outputs[1].tokenGroupId == tokenid);

        /* Calculate Provider commission amount (received). */
        int commission = (sellerPayout * fee) / 100;

        /* Validate and handle commission payout. */
        if (commission > 0) {
            /* Create Provider bytecode. */
            bytes23 providerBytecode = new LockingBytecodeP2PKT(provider);
            require(tx.outputs[2].lockingBytecode == providerBytecode);

            /* Verify Provider payout amount. */
            require(tx.outputs[2].value >= commission);
        }
    }
}

👆️ working on a semi-DEX contract now (does a one-way asset trade)