limit product quantity

Hi,
is there a way to limit the product quantity available to be sold? I put 1 in quantity while creating a new product, but the customer still can add more than 1 item of that product to his shopping cart.
What I need is I only want to sell a 1 unique item of a product, then nobody shouldn’t choose more than 1 item while it’s already not been sold. From the moment the only item put to sell has been sold, then nobody can any longer buy this product because it’s no available. How Can I achieve this behaviour and show a message “Sold out” (or similar) in product description from the very moment it’s no longer available???

Thank you.

http://wordpress.org/extend/plugins/ready-ecommerce/