Items held in shopping cart for time limit?

Hi,

Most of the products on this particular MarketPress site I am developing, only have one item in the inventory. This is because they are individual and unique jewellery pieces.
Everything is working on the site except that when a visitor adds items to their shopping cart, it is unavailable for purchase by anyone else.

This is fine initially, but how can the time limit be set to hold the items in the shopping cart?

Thank you if anyone can offer some advice.