In view of the huge risks brought by transaction fraud， handover irregularities and other issues in bulk stock online trading， a long-term traceable online trading mechanism was proposed to achieve more reliable bulk stock trading， in order to realize the authenticity and anti-tampering of information and the credibility and anti-fraud of process. Firstly， combined with blockchain， an online trading framework based on the idea of "application-verification-record" was proposed， and the smart contracts were used for multi-party supervision and detailed records for each stage of the trading process. Secondly， to guarantee the authenticity of commodity information， for bulk stock with texture features on its appearance， the commodity fingerprints of the bulk stock were extracted and verified based on the Local Binary Pattern （LBP） algorithm. Finally， to ensure the credibility of the handover process， a standardized handover method of commodities was proposed on the basis of environmental fingerprints. The above trading framework， commodity appearance fingerprint extraction and verification algorithm， and standardized commodity handover method were used jointly to constitute the online trading mechanism. The analysis results show that the proposed trading framework can avoid most of the frauds from the perspectives of user selection and process specification and can identify single-party and two-party frauds occurring in the transactions. The experiment results based on the real log image data show that the proposed commodity appearance fingerprint extraction and verification algorithm can judge different images of the same commodity with 94.00% accuracy and distinguish images of different commodities with 78.30% accuracy. The system performance test shows that the delay of each stage of the proposed trading mechanism is within an acceptable range， and meets the requirements of online trading.