sell, be popular
售 centers on the act of selling, with the extended idea of something being popular or in demand because it sells well.
售 combines 隹 (short-tailed bird) and 口 (mouth), likely suggesting a bird calling out, which was extended to the idea of hawking goods or selling. The exact historical development is uncertain.
A bird (隹) opens its mouth (口) to chirp loudly, like a vendor calling out to sell goods. The more it chirps, the more popular its wares become.
For シュウ, imagine a shoe salesman: shoe -> シュウ, and he's selling shoes so well they become popular.