The only real way to gain any significant share of the market is by under cutting the price of the competition. The power of the suppliers is strong. Retailers must maintain good relations with their vendors. If their reputation is soiled with the suppliers, then they risk losing their source of merchandise and will be driven out of business. The power of the buyers is also strong.
Retailers are at the mercy of the consumer's demand and preferences.
Currently consumers find shopping to be a chore. The convenience of the general merchandise store allows customers, to buy more items in fewer trips. The power of new market entrants is weak. The market is extremely easy to enter into, but it is hard to compete at the levels of economies of scale that the larger chains possess. The power of substitutes is very strong.