Sorry for coming to this discussion so late, but I can offer at least a partial solution. We do not have anything to offer on the listing side, but the Stone Edge Order Manager (stoneedge.com/ordermanager.htm) should be able to handle most, if not all, of your back-end order management requirements. The Order Manager can import orders from Amazon, eBay and about 40 popular shopping cart systems. There is also an API that lets you integrate it with custom shopping carts. It is designed for multi-channel merchants: you can import orders from any combination of sources into one database, where they will share the same customers and inventory, while retaining the branding of your various channels. The Order Manager is used by over 2,500 merchants. Most of them handle 10 to 500 orders per day, but we also have 11 users in this year's Internet Retailer Top 500, shipping as many as 4,000 to 5,000 orders per day.
And sorry, Linn Systems, but we do give telephone support for our order management system. Yes, it is expensive (currently 10 full-time employees for tech support alone), but we find that it is the only effective way to give our users the level of support that they need. And our prices are much closer to yours than to SAP's!
I will be glad to answer any questions about our systems here in this forum, or give us a call at 610-994-3699.