Thanks for the feedback. I like certain aspects of Numista, and it's probably the best online tool that's currently out there. But I also think certain aspects of it could be much better than they currently are. Off the top of my head, I think the user-interface design can be confusing, the grading options it gives are limited, and while it lets you add coins to your collection, it doesn't really allow for the adding of much information that's specific to your coin (pictures, where you bought it and for how much, etc.). It also doesn't have any built-in tool to estimate the value of your coins / your collection. It's not that numista isn't a good tool for a certain type of user, it's that it's an incomplete tool (which is my challenge with most of the tools out there right now - they're good for certain use cases, but not for others).
As for the ebay idea, I personally like that idea a lot. I'd have to see how structured the searchable data is in ebay, but that's one type of feature I think lots of people would find useful.
Thanks again,
Jack