You can't compare Walmart to Amazon, Walmart is a northamerican thing, Amazon is global. Walmart has zero influence in the prices of the tomatoes grown in Europe.
If you limit it to only retail, Walmart probably comes out ahead.
Alas, them deciding to shut-down Book Depository means I can't purchase English/American books anymore free of customs-related taxes (I live in a Eastern European EU county), which makes me not purchasing those books at all (in many cases the taxes would end up doubling the purchase price, so it's not worth it). I'll never forgive Amazon for that.