Online buyer coalition formation problem is an applica- tion of e-commerce and distributed agent technology. Most existing works in this topic involve social utility based ap- proaches that assume the agents' utilities to be transferable. However, we argue that there are situations where the trans- ferable utility model is not well suited for the problem, and that coalition stability is a more important solution concept than social utility. In this paper, we study the problem from a non-transferable utility approach where the focus is on achieving stable solutions in term of the core and Pareto ef- ficiency. A new distributed mechanism is proposed where agents are allowed to propose incremental improvements toward a stable solution. We show by experiment that our mechanism is able to reach core-stable solutions in over 99% of the cases, which suggests a big improvement over the existing approaches.