Such selling at a loss is really weird from an anti-trust perspective. As a worst case, it allows a company to 'invest' in market share by selling at a loss to strangle the competition. Once this is done, they can entrench themselves and raise prices. If you only look at anti-trust as consumer protection, the above scenario only becomes problematic when its to late.

That said, lines are hard to draw. When is a company lowering prices to strangle the competition and when is the competition being strangled a side-effect of efficiency? Is it okay if amazon intends to always stay cheap, and use retail as a loss-leader? What should happen when supermarkets dampen price fluctuation, taking a loss when prices spike to appease their customers.

It's a hard problem, but we should be careful about companies operating at a loss just to strangle their competition. However, should this come at the cost of companies that strangle their competition by just being more efficient?