Calvin the Salesman is a running gag in which Calvin sets up a booth (usually made from a cardboard box) to sell some product. Calvin has used many different ideas over the years to try to earn money, but they were all unsuccessful.
Insurance
This booth retailed insurance at 50¢ apiece. When Susie made light of it, asking who would buy insurance from him, Calvin shot her with a slingshot.
Family Car
Calvin, upon realizing that he lacked money, put up the family car for sale. The price was unspecified, but said to be low, and certainly less than the car's value given Calvin's nebulous conception of money and the economy.
Lemonade
Calvin has two several lemonade stands over the years. The first time he sold it for five dollars a glass—during winter. He claimed that it had "all-natural refrigeration", which albeit true was no selling point. Another time, he was selling a single glass for an otrageous $15, and his concoction was really just a lemon dropped in a pitcher of sewage. Susie Derkins tried to tell Calvin about this, but Calvin was able to justify the product and pricing through economical phenomena.
A swift kick in the butt
This was Calvin's least successful business. He didn't sell one, and seemed fairly surprised that it wasn't successful, since he thought everyone he knew needed one.
Life
Once, Calvin set up a booth labeled simply "Life 5¢". Susie gave Calvin a nickel, only to be told that she would receive nothing. "That's life!" Calvin told her. She responded with crushing the booth, and knocking Calvin to the ground.
Great Ideas
Calvin once gave out great ideas (at first at $1 each, later reduced to 25 cents). Calvin's mom bought one, and the idea Calvin gave her was "Buy some more!"
Happiness
Calvin made a "Happiness 10¢" booth, where it's basically a water balloon in the "kisser." When Hobbes asks, "Whose happiness is this?", Calvin said, "Who went to all this trouble?!?"
Candid Opinions
Calvin made a "Candid Opinions" booth. When Susie walked by, Calvin called her a "bat-faced, bug-eyed, booger-nosed, baloney-brained, beetle-butt." Laying on his crushed booth, Calvin remarked, "This volunteer social work just isn't for me."
Frank Appraisal of Your Looks
Calvin once made a "Frank Appraisal of Your Looks 50¢" booth. When Hobbes asked him if he had sold any, he said, "Not so far, but I'm giving out lots of free samples!"
Suicide Drink
In a Sunday strip showing a summer day for Calvin and Hobbes, the two shown sitting at the box with a pitcher filled with sludge, much like with the Pitcher of Plague (see below). The box read "Suicide Drink 25 cents". There was no dialog in this strip.
Calvin's Curative Elixir and Pitcher of Plague
Calvin presented a solution called "Calvin's Curative Elixir" to Hobbes. However, the latter argued that it was not a curative elixir of any kind, observing that it was just water from the drainage ditch that had leaves in it. Calvin tried to convince Hobbes to let him sell it by saying that it was "fortified with chlorophyll", but Hobbes managed to convince him to change the name, and soon they were selling "Pitcher of Plague: Calvin's Debilitating Disease Drink" and charging a dollar not to have any.