What kind of transaction was going on between Manuel and a man at College Park in Call of the Wild?
The transaction going on between Manuel and the man at College Park was the sale by Manuel of stolen goods - namely Buck - for eventual resale on the black market.
Manuel, one of the gardener's helpers on the estate of Judge Miller, where Buck lived, had run up gambling debts and was having problems fulfilling the needs of his wife and many children. One night, while "the Judge was at a meeting of the Raisin Growers' Association and (his) boys were busy organizing an athletic club", Manuel brazenly stole Buck, surreptitiously leading the valuable dog to a "little flag station known as College Park", located near the ranch. There, Manuel met with "a solitary man", and after talking for a short time, money was exchanged. The man was a dog-breaker by trade, and it was to him that Manuel sold Buck for a hundred dollars cash.
The dog-breaker muscled a furious Buck into the baggage car of a train, and accompanied him to San Francisco, a task for which he was paid fifty dollars. He turned the dog over to a saloon-keeper on the San Francisco waterfront, where the animal was brutally beaten and locked in a crate. Buck's forced journey continued via wagon, truck, a ferry steamer, and finally an express train, to a destination in the Klondike, where he was sold on the black market to Francois and Perrault, who worked as mail drivers in Northern Canada (Chapter 1).