Abstract: Interaction and transaction between merchants is very common process in traditional or physical market. By using this interaction, merchant can provide product in more quantity even their stock is less than the quantity that is sold in the transaction by using other merchant stock. Unfortunately, this interaction has not occurs in the online marketplace system yet. Although, the online marketplace system has similar characteristic with the conventional market which there are more than one merchant that sell same product, there is not any interaction model that has been developed, so that, these merchants can interact to each others. Based on this problem in this study, we propose new inter merchant negotiation model that can be implemented into online marketplace system. So, merchant can sell more quantity by using other merchant’s stock. This negotiation model is developed by using stochastic approach. In this research, there are two models that are developed: serial negotiation model and parallel negotiation model. These proposed models then are implemented into the online marketplace inter merchant simulation application, so that, the model performance can be evaluated. Based on the test result, there are some research findings. First, the parallel negotiation model performs a little bit better than serial negotiation model in producing lower total transaction value. It means that by using parallel negotiation model, buyer pays lower because he gets lower deal price. Second, the number of sellers has negative correlation with the total transaction value. Third, the seller’s stock quantity has negative correlation with the total transaction value. Fourth, the requested product quantity has positive correlation with the transaction value.