Valuation is the fair value of a firm and is calculated using estimates and predicting the future cashflows and discounting them to today's date. There are various ways of valuing a company and they can be done for those companies that do not pay dividends or aren't even profitable because its all about forecasting the numbers and the valuation will be a product of these forecasts.

Companies like Amazon are valued using the sales that occur however they provide deep discounts in order to entice customers and hence they are not making overwhelming profits due to which athey are not bale to pay dividends. The valuatiionn is justified because it depends on the amount of sales they have made which is huge.