I priced an item at 500k Professor New Foxy Shoulder Guard

but it says it's display only? Why won't it go in for a sell?

50 million is the limit for shops. Shops do not accept any punctuation in prices: if there's any symbol other than numbers it won't work.

(this is great for me because my cat sometimes thinks I need help when pricing)

Sometimes the people can have a gallery instead of a shop! This is where you display you items but you don't sell them.

Aww Kitty's just tryin to help! lol