Ben,
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.