I have a site that I built using cubecart [url removed, login to view] and the business model requires a modification that I can not seem to locate on the net - a "Make an Offer" module.
The site is for selling collectibles and there are some applications where it is ok to post a sell price with Add to Cart, but for most items it would be better to post a "Make an Offer" link on the item page as well as the Add to Cart button.
So there would need to be an option in the admin when you enter the product to choose which way it is listed.
Each item should have a "minimum accepted price" field as well so if an item needs to get at least $20 and someone makes an offer of $15 you can quickly know if it can be accepted or not, or at least have the option to take a lower offer. This would work like the "reserve" feature on Ebay.
Once someone makes an offer, there should be a page in the admin that lists all the offers that are "pending". An email should be sent for each one to the admin (or some other email that can be setup in the admin panel) with a link to the pending offers page.
At that point there should be an option to accept the offer, email the person making the offer, Negotiate the offer, or reject the offer. Each of these options should include a "comments" field so you can add comments which would get emailed back to the person making the offer.