When you create or configure a product list, you can set the quantity of products you wish to sell on ricardo.ch by editing the "Sales options" tab.
This quantity is not reserved for ricardo.ch, it just indicates to ricardo.ch that you want to sell a certain quantity of products.
When an order is passed on ricardo.ch and this order is then created into Magento, the normal Magento stock management procedure will apply.
Before release 1.4.x
However, if an order is passed on Magento and the inventory decrease in your stock on Magento or ERP level (or else), ricardo.ch won't be aware of the change and will keep the quantity defined while you listed it into ricardo.ch.
A workaround exists if it's critical for your business to have to deal with missing or not enough products in stock is to use the ricardo assistant located in Magento menu ricardo.ch > Ricardo Assistant to edit manually the quantity of a product.
In a future version will provide a better solution for the synchronization of the stock from Magento to ricardo.ch.
New in release 1.4.x
The stock management works in both direction, it means sales done on Magento will reduce the quantity on ricardo.ch side if the product stock reduce under the quantity published on ricardo.ch.
Quantity sold on ricardo.ch will be also decreased if the product stock has been updated while editing the product in Magento Backend and the quantity is below as the one published on ricardo.ch.