It is unknown how long Horace Scope has worked on Star Shopping Network, but he first appears after Tim creates, completes and, in a freak of human error, subsequently breaks Dorothy Ann's birthday present. When he appears on TV, he offers to sell the naming rights to three different stars, a T Tauri star, a main-sequence G-Type star, and a Red Supergiant star at seven dollars each.
Horace Scope has a working knowledge of stars and knows, despite the naked eye, that they're all different in some way, especially the young, T Tauri star in the constellation Taurus, which he pitches first. Of course, for his customers to see the star close up, they'd have to travel faster than the speed of light, and it would still take a lifetime to reach it, so with no way of a guarantee on his products he banks on his customers calling him on impulse instead. Unbeknownst to him, Ms Frizzle and her class have a Magic School Bus capable of reaching the stars and are scrutinizing his "products" as he speaks.
When it turned out that the T Tauri star was unpopular, being young and unstable, Horace Scope moved on to the next, a G-Type star nearly identical to Earth's sun in age and with its own system of planets. He successfully sells that star to Janet, much to the chagrin of Ms Frizzle's class (especially Arnold).
The third star on his list was a Red Supergiant. Knowing this star was at the end of its life, Horace Scope puts a fifty cent discount on the star. After which, he gets a call from Tim, who buys the star and names it for Dorothy Ann.
"...and not a moment too soon." - Horace Scope
Indeed, the star goes nova almost immediately after and Horace Scope, in effect, cheats the class out of six dollars and fifty cents. What he doesn't know is that, at this moment, Ms Frizzle and her class are recreating the star out of the supernova remnants, having learned that new stars are made from the remnants of old, massive ones. With the help of the bus, they accelerate the process and succeed in creating a new star.
Shortly after, Horace Scope discovers the new star, unaware of the Frizz and the kids' involvement in making it. Horace Scope attempts to market the new star. However, the class calls him out on it, explaining how stars are created, chastising him for selling them a dying star and arguing that the new star is, in fact, the same star he sold to them, having been recreated from the remnants of the latter. Horace Scope, sweating from guilt, refrains from reselling the star and Liz turns the TV off on him.