At the heart of the Lio strip that ran on 4/28/16 is a simple bit of word play. Mark Tatulli's gag uses two different meanings of the word " balloon" for the humorous payoff.
It works. But there's a little bit more to unpack here, because that's what makes this sequence even more enjoyable for serious comics fans.
First, understand that Lio is a strip in which no one talks. Or rather, a strip with no word balloons. so of course his word balloons are for sale -- they're useless in his strip.
Second, note that the customer contemplating Lio's wares is Nancy -- a character whose strip is dialogue-heavy. Nancy would definitely have a use for extra word balloons.
Third, note the variety of Lio's wares. And take a step back to realize that we intuitively know what these conventions mean. There are the traditional round balloons, with tails pointing in different directions. If two people are talking in the same panel, each balloon would have a tail going to a different speaker -- hence, you'd need both a right-pointing and a left-pointing word balloon.
See the spikey one? That indicates shouting, or a loud noise. The square one at the end is often used to depict emotionless or mechanical voice, like a robot, speakerphone, or computer navigation.
Comic readers know what these indicate, without ever consciously noticing the word balloon at all. In this case, though, there are no words to distract us, and we're left to contemplate the shapes.