Do the shops ever restock their items?

They sure do. On the first of each month.

they do, every month basically.
the number of items restocked, depends on how rare they are..
and things like that

Edit: Got ninja'd :|
gg Fee.

Thank you guys

How much does it stock up?

Gin: Depends on the item, they don't want to imbalance the market, the rarest items will be restocked the least.

You can find all information related to the restocking of items here.

Hope this helps.
sorry for my bad english

*Below the first post, there are some questions answered by the moderators.
Some of these doubts can be yours.