你不用看店家贴什么告示，那个不是重点。一个你未必能找到人证明，二来即使有人证明，看店主不想陪的样子，那个人的personality和credibility都有可能被challenge。你的论点我觉得应该不是看那告示是什么时候贴出来的(按店主的思路行事)，而是不管有没有那个告示，店主都不可避免的对你的物品的遗失负有责任。
你的情况我觉得属于bailment。To transfer custody and possession of your personal property to another individual, and that individual accepts custody, there is said to be a process that has occurred known as "bailment".
在这种情况下， While the individual or company may claim no liability for damage or loss of your personal property, they may have some liability to them based on the court's findings of implied bailment principles.
