A product like EzSupport, which also offers Ebay messaging integration may also be an option. See http://www.hostedsupport.com/index.cfm?fuse=products.ezSupport

or it may be something that one can learn from.

Or there is open source product vTiger at http://www.vtiger.com/

 

2) As to a perceived 'lack of demand', I believe you're in error. Maybe EI asked the wrong customers?  No online store with common sense would want to continue using a separate email client, with continued redundancies so work duplication, if he can avoid it. There is enormous demand for integration of data and emails, because of its large productivity increase and lowering of costs. The proof is in the fact that many of the most successful software and online sites in e-commerce offer this integration, like:

 

- Interspire (shopping cart)

- Viart (shopping cart)

- Netsuite (shopping cart)

- Americommerce (shopping cart)

- EcBuilder (shopping cart, created by one of leaders in contact management software, Maximizer)

- Auctionwizard2000 (auction management for Ebay sellers)

- Ebay (e-commerce site which has its own messaging system and various 3rd parties also provide integration with it via its existing API)

- Bonanzle (e-commerce site which has its own messaging system, due to open its site with 3rd party API in coming months)

 

Note:

Please take into account that I am really talking about something more than just a marketeer tool. That side of CRM is just a small part. The most important part is to offer a full 'communication center', one place where all communications with customers can get managed through their entirely lyfecycle, from creation till deletion or archiving.