Speaking at the 2019 Berkshire Hathaway annual meeting of shareholders, Warren Buffett on why Amazon could be a value stock:

Yeah. It’s interesting that the term “value investing” came up. Because I can assure you that both managers who — and one of them bought some Amazon stock in the last quarter, which will get reported in another week or ten days — he is a value investor. The idea that value is somehow connected to book value or low price/earnings ratios or anything — as Charlie has said, all investing is value investing. I mean, you’re putting out some money now to get more later on. And you’re making a calculation as to the probabilities of getting that money and when you’ll get it and what interest rates will be in between.

There are many ways to value a security, but as Buffett explained last year, the most important thing investors need to consider is the cash flow characteristics,

In the end, it all goes back to Aesop… a bird in the hand is worth two in the bush… And when we buy Amazon, we try and figure out… whether there’s three or four or five in the bush and how long it’ll take to get to the bush, how certain he is that he’s going to get to the bush, you know, and then who else is going to come and try and take the bush away…the basic equation is that of Aesop. And your success in investing depends on how well you were able to figure out how certain that bush is, how far away it is, and what the worst case is, instead of two birds being there, and only one being there, and the possibilities of four or five or ten or 20 being there.