Is Best Buy an authorized Apple reseller?

I am planning on buying iPhones as presents for 5-6 people, because they have birthdays. They are actually my students, and they really really wanted an iPhone 6s. One is stuck on iPhone 4s, one on iPhone 5, 2 on iPhone 5s, and 2 on iPhone 6. They really really wanted an iPhone 6s, so I buy them 64GB. Should I buy one on Best Buy, because it costs $799 each device. They all want unlocked, and Apple tend to sell them for $749. On Amazon, some are over $800. And should I buy ones with no box included, and iPhone only? On eBay, some iPhone 6s 64GB cost $900. AT&T said it for $749. Verizon, Sprint, and T-mobile also set it for $749. Walmart has the Straight Talk version, and should I buy that one, or it is not really authorized?