To view all items currently in the shop, use the command /shop.

If you want to sell an item in the shop, hold an item in your hand and use /sell [price]. The price must be a positive number, and suffixes such as "k" "M", "B", "T", and "Q" are supported too.
Examples: /sell 1200, /sell 1,2k

To get your items back from the shop (including the expired ones), use the command /listed.