HELLFIRE Slavishly copying Amazon's model? So long as 3rd party pays a certain percentage to Walmart, Walmart doesn't care? And it's one more line item propping up the bonus checks to the C-Suite?
Precisely this! Far too many companies today - particularly large companies - are all about just their quarterly bottom lines. Long-term company reputation matters little to hired managers who plan on moving up to an even bigger job title somewhere else in 4 years and who seek huge bonuses in the meantime.
The entire "3rd-party seller" concept is a way to export business inventory and support costs to someone else whilst skimming transaction/preferred-placement money off the top. Unfortunately, hosting 3rd-party selling also holds the host company's reputation over those sellers like an umbrella. Unless the host company rigorously polices any 3rd-party sellers, bad things will eventually happen... but policing costs effort (which means money), and that can reduce bonuses in the short term - hence you get what we're seeing now, all too often.