I’ve had a quick look at your website and it appears impossible for search for what I’m looking for in any way at all. I’ve entered a book title, an author’s name or a genre into the search bar, and literally nothing happens.
I suspect that people visit secondhand bookstores online not to browse, but rather looking for specific books or authors / genres of interest. Unless you can make this happen, it seems unlikely the site will generate any sales at all.
Selling stuff online is all about understanding why people are seeking to buy online – maybe you could talk to your existing in-person clientele to understand how they would use an online secondhand bookstore?
Hope that helps…