Mine is when marketplace sellers go "oh, sorry, we got your order but this is out of stock" or worse, "we mispriced it, it's actually $100 more we cancelled you

Mine is when marketplace sellers go "oh, sorry, we got your order but this is out of stock" or worse, "we mispriced it, it's actually $100 more we cancelled your order". Zero repercussions for staying a marketplace seller, no repercussions for the kind of behaviour the store itself would never accept.