A man and a woman want to buy a planet. The woman opens a box and the salesman shouts: "Don't open it, it's a black hole!" and they all fall into it.
You may be thinking of Star Salesman, an episode of a science video series Universe & I produced by Kentucky Educational Television back in the '70s. Unfortunately, there's very little information about the series on the Internet. I remember seeing the episode a few times as a kid.
The story is modeled on the trope of a young couple starting out as a family and visiting a car dealership to buy their first car. It's set in "Star Mart", a store that sells stars. The couple want to buy their first star and talk to a fast-talking star salesman, played by Carl Ballantine. The salesman shows the couple different stars and answers their questions. The man asks about a blue giant, and the salesman says they're flashy and burn out too fast. The salesman tries to steer them toward a nice main sequence star because they're reliable and last a long time. That sort of thing.
Along the way, the characters talk about the different kinds of stars, how they form, how they're maintained by fusion, and how they end. One specific bit you may recall is about how fusion in larger stars produces a succession of heavier elements. The salesman describes--I may be misremembering the science here--that the star under discussion fuses hydrogen into helium, then helium into something heavier, and so on up to iron. The woman(?) asks why the process stops at iron, and the salesman says it's because fusing iron uses more energy than it produces.
I have a vague memory that the story ends in the way you describe. The couple has settled on their star and the salesman is congratulating them on their choice. The woman sees a display of boxes apparently containing black holes, wonders what's in them, and opens one.
As I said, there's very little information about the series or this episode online. This question asks about Mind Slaughter, another episode in the series. An apparent Star Trek fan has preserved Interior Motive, another episode which happens to star Leonard Nimoy.