I'll throw this out there because you might consider it relevant.
I recently learned that Abe HomeBase is pretty much the de facto standard book inventory software out there. It seems there's little motivation to search for other platforms because you have to use HomeBase to get listed in Abe (as I understand).
I learned all of this when a friend was showing me HomeBase at the bookshop he worked at... and in particular its mind-numbingly slow live search system.
Take Google autocomplete, make it blocking/non-asynchronous (it freezes while it gets your result), and slow it down so EACH KEYPRESS takes FIVE SECONDS to acknowledge. That's HomeBase search.
I have no idea how you would go about fixing this, but I thought I might mention it in case this direction sounds interesting. I'm not sure, but I get the impression this is one of those areas with no movement or competition, and it just needs someone with a bit of motivation to come along and turn it upside down.
If you like I can put you in touch with my friend; he's very technically minded, so you could very likely get a lot of in-depth info about how the program works and what needs to be done to replace it.