The sign for "store" is made by bending both wrists and pointing both hands down.
Pivot both of your hands toward and away from your body.
Use a double motion. If you use a single motion, this sign means "sell."
I've noticed though, that quite a few signers use the single motion to mean
"store" and use the double motion to mean, "selling."
This is opposite the "noun/verb pair" principle. So, even though
I still teach that the "right" way to sign "store" is with a double
motion, I also encourage my students to be flexible in the "real world."
(The fingertips swing forward and back a couple times.)