Diseconomies of scale in fraud, spam, support, and moderation

If I ask myself a question like "I'd like to buy an SD card; who do I trust to sell me a real SD card and not some fake, Amazon or my local Best Buy?", of course the answer is that I trust my local Best Buy1 more than Amazon, which is notorious for selling counterfeit SD cards. And if I ask who do I trust more, my local reputable electronics shop (Memory Express, B&H Photo, etc.), I trust my local reputable electronics shop more. Not only are they less likely to sell me a counterfeit than Best B...