Tinkering around with the search tools, and a few things I'm noticing.

I feel like Availability should default to In Stock, but maybe that will come once you have more inventory on the site.

I haven't figured out exactly how to reproduce it (sometimes it's when I go to another page of the search, or sometimes it's when I change filter settings), then I get:

"Internal server error Error loading dynamic filter options

The site administrators have been notified."

When browsing "Other versions", it seems like it would be nice if it limited it to in-stock items only, or at least showed me the lowest available price for each entry so that I can see if there's stock there.

Overall it looks like a great start to the tool -- well done!