Some have dimensions that don't fit your eyes and are uncomfortable to use. For example, oversize oculars can give wide field views but if your eyes are close set, you will not be able to center your pupils in the ocular. Extreme eye relief for eyeglass wearers forces you to hold the binocular suspended in front of your eyes. If children are going to use them, remember their small interpupillary distance means that they will not be able to see through both sides of most binoculars. There are also the physical dimensions of weight and size that are hard to judge without a trial.
Light weight is always more comfortable for extended use but usually means a less rugged glass.
Thus, mail order is hazardous unless you have tried the binocular you are ordering. Several local stores have selections of binoculars. There are camera stores in New York with extremely low prices, for the manufacturers sell to them at huge discounts, allowing them to sell for less than the local businesses pay at wholesale. The first important choice to be made is the size of the objective lenses, and for astronomical use they range between 35mm and 100mm. 35mm is a vast improvement over unaided vision, 50mm transmits twice as much light as 35, 80mm are difficult for almost all people to hand hold, and 100mm are very expensive and heavy.