We analyze the impact of purchasing alliances on product variety and profit sharing in a setting, in which capacity constrained retailers operate in separated markets and select their assortment in a set of differentiated products offered by heterogeneous suppliers (multinationals vs. local SMEs). Retailers may either have independent listing strategies or build a buying group, thereby committing to a joint listing strategy. This alliance may cover the whole product line (full buying group) or only the products of large suppliers (partial buying group). We show that a buying group may enhance the retailers' buyer power and reduce the overall product variety to the detriment of consumers. Our most striking result is that partial buying groups do not protect the small suppliers from being excluded or from bearing profit losses; they may even be more profitable for retailers than full buying groups.