Why were Buck and his Mates sold before having a chance to rest and recover?

There was no time to rest the drivers or the dogs, as upon their arrival the drivers were given official orders to move the mail that was piling up. The dogs couldn't go on, they would have been worthless; so new dogs were had, and Buck and his mates were sold.

"The drivers confidently expected a long stopover. Themselves, they had covered twelve hundred miles with two days' rest, and in the nature of reason and common justice they deserved an interval of loafing. But so many were the men who had rushed into the Klondike, and so many were the sweethearts, wives, and kin that had not rushed in, that the congested mail was taking on Alpine proportions; also, there were official orders. Fresh batches of Hudson Bay dogs were to take the places of those worthless for the trail. The worthless ones were to be got rid of, and, since dogs count for little against dollars, they were to be sold.

Three days passed, by which time Buck and his mates found how really tired and weak they were. Then, on the morning of the fourth day, two men from the States came along and bought them, harness and all, for a song. The men addressed each other as "Hal" and "Charles". Charles was a middle-aged, lightish colored man, with weak and watery eyes and a mustache that twisted fiercely and vigorously up, giving the lie to the limply drooping lip it concealed. Hal was a youngster of nineteen or twenty, with a big Colt's revolver and a hunting knife strapped about him on a belt that fairly bristled with cartridges. This belt was the most salient thing about him. It advertised his callowness--a callowness sheer and unutterable. Both men were manifestly out of place, and why such as they should adventure the North is part of the mystery of things that passes understanding."

THat is because the drivers, who worked for the postal service, were called elsewhere to deliver letters and packages for men in the goldmines. They purchased fresh dogs and sold Buck and the wary pack for a song to Hal and Charles.