How does Mr. Shelby describe Tom? Why does he think Tom is valuable slave?

The first scene of Uncle Tom's Cabin depicts a conversation between two gentlemen, Mr. Shelby and Mr. Haley. Shelby is loathe to part with Tom, who has been his loyal servant since boyhood, but nevertheless he tries to persuade the trader of Tom's qualities- religious piety, honesty, and sensibility- so he will not have to sell more than one slave. Tom is a "loyal" slave and is able to keep other slaves in line.