Abstract: In order to maintain the growth of retail sales, e-commerce platforms regularly organize online promotional activities, and third-party merchants on the platform actively participate in them. Promotional activities can attract a large number of users in a short period of time. For example, during the 2023 Tmall Double 11 promotional period, the cumulative number of visiting users exceeded 800 million, with more than 20 million new purchasing users and over 140 million orders added, achieving market expansion during the promotional period. However, some merchants may experience a decrease in product sales after the promotion. For instance, Amazon merchants saw a drop in order volume after the Prime Day promotion. Therefore, promotional activities initiated by the platform not only increase demand during the promotional period, helping merchants to expand market share and attract new customers in the short term (promotion expansion effect), but also have an impact on the long-term product demand of merchants (long-term effect of promotion). The short-term expansion effect during the promotion and the long-term effect after the promotion add complexity to the decision-making process of the platform and merchants, causing merchants to continuously adjust pricing strategies during the promotional cycle, leading to “false promotions.” The internal reasons and mechanisms for merchants to engage in false promotions are not fully understood at present. Based on this, this paper analyzes the impact of the expansion effect and long-term effect of promotions on the strategic choices of platforms and merchants, reveals the internal mechanism of merchants' false promotions, and helps platforms and merchants to formulate more reasonable promotional strategies and improve promotional effects.
   This paper starts with the pre-promotion period as the research starting point, and based on previous research, constructs a dynamic game model of the platform and third-party merchants during the pre-promotion, the promotional and the post-promotion period. At the same time, considering the short-term expansion effect of the promotional activities and the possible positive long-term effect, negative long-term effect and no long-term effect in the post-promotion period, the paper discusses the impact of the expansion effect and long-term effect on the promotional strategies of the platform and third-party merchants. This paper attempts to answer the following four questions: (1)How should e-commerce platforms and merchants formulate different stages of promotional and pricing strategies when there is an expansion effect and long-term effect of promotion? (2)How do the expansion effect and long-term effect affect the optimal strategies of platforms and merchants? (3)How do we expose the internal mechanism of merchants’ “false promotion” behavior? (4)How do e-commerce merchants interact in price at different stages?
   The research findings find that, during the promotional period the promotional strength provided by the platform increases with an expansion effect and an increase in the commission ratio. When there is a significant negative long-term effect, the platform tends to adopt a conservative promotional strategy. Merchants will reduce the price in the pre-promotion period when the market expansion effect is small and there is a negative long-term effect, and increase the price in the pre-promotion period when the expansion effect is large. Merchants will increase the price during the promotional period and make the price higher than the basic price in the no-promotion situation in two cases: first, when the long-term effect of the promotion is negative; second, when the long-term effect is positive and relatively small, and the expansion effect is moderate. When the long-term effect of the product is negative, it will be more likely to produce false promotions, and an increase in the expansion effect will increase this possibility. It is also found that when the expansion effect of promotions is significant, false promotions can also reduce consumer surplus.
   This paper enriches the theoretical research related to e-commerce promotions and provides a basis for decision-making during the promotional period for e-commerce platforms and third-party merchants. In the future, platform promotions with returns will be considered; at the same time, the competition between platforms and merchants will be included in the research context.

Key words: platform promotion strategy, expansion effect, long-term effect, false promotion

摘要： 为揭示商家“虚假促销”的内在机制,构建了平台与第三方商家在促销前期、促销时期和促销后期的三期动态博弈模型,比较了不同时期的均衡结果,分析了促销扩张效应和长期效应对平台和商家策略选择的影响。研究发现,在促销期间,平台提供的促销力度随扩张效应增强和佣金比例提高而增加。当存在显著负长期效应时,平台会降低促销力度。对于第三方商家而言,当市场扩张效应较小且存在负长期效应时,商家会在促销前期降低价格。相反,当扩张效应较大且长期效应为正时,商家会在促销前期提高价格。商家会在两种情形下进行“虚假促销”:一是当促销长期效应为负时;二是当长期效应为正且相对较小时,并且扩张效应中等时。长期效应为负的商品更容易产生虚假促销,且扩张效应的增大提高了这种可能性。同时发现当促销扩张效应较大时,虚假促销也会降低消费者剩余。

关键词: 平台促销策略, 扩张效应, 长期效应, 虚假促销