Forum OpenACS Q&A: Response to online reservations and payments
the current ecommerce module is ill equipped for customers to place orders for items that are not yet in the product catalog. You might want to take a look at the brief description of a new Merchant System that Tom Jackson is working on. In his design allows customers add products to their order, even if they do not already exist in the system. This allows customers to better communicate what products they need to purchase.
Then there are other considerations which don't fit the ecommerce package well. The quotes for the item (or service) would have an expiration date/time. The current ecommerce package is exclusively geared towards selling products from a catalog of a single vendor.
The OpenACS toolkit as a whole offers an excellent platform to build the system you are looking for on. The payment service contract would tackle the financial transactions while other (new) packages could meet some of the other challenges.