The problem with used books is, whether or not they have a barcode — and the ISBN system was not in play until the late ’70s; even then, many mass market paperbacks do not have a barcode that corresponds to their ISBN when scanned, for… some insane reason I’ve never been able to figure out — that doesn’t matter. Used bookstores price their OWN stock; they don’t sell books at their original publisher-set price. We pencil all our prices in as we buy stock, then shelve it. As soon as you start adding any sort of computer inventory system to that very simple one, things get way, way more complex and time-consuming (for really no good reason — many of our titles we sell for $5 or less, and would not be worth the time, cost, or effort for us to ship out). Make more sense?
Tl;dr secondhand bookselling is really, really not designed to easily sync up with online bookselling. This is why people tend to either specialize in one or the other, but rarely blend the two (it’s a huge headache).