Capitalism works under a lot of assumptions, one of them is no friction with information. This particular story is just taking advantage of a small, imperceptible (to the rentors) risk capital gap, but some savvy and attached fans saw it and wanted to take advantage of it.
It's just like a lot of microfinance in say a poor struggling neighborhood in Bangladesh. There's money to be made there, but most traditional bankers or participants would ignore it or be blind to it. When socialism steps in is not when it's able to cover that gap, but when it has the foresight or wisdom to know why that gap should be covered. That wisdom is something that doesn't always happen in less than ideal capitalist systems.
In addition to being a bookshop, Borderlands is a cultural hub for SF/F readers in the Bay Area. As such, it's not simply selling SKUs at the cheapest viable price, it's providing an intangible curatorial service. Amazon may crowdsource reader reviews but they're unlikely to be as insightful as suggestions by enthusiastic staff who know what they're selling, and the usual recommendations engines are less effective than one of those staff engaging in Q&A with a customer to find something new that they might like.
(And this is before we get into things like the cafe side of the business, or the author readings/signings. Disclaimer: I am an author and I occasionally do events there.)
This exchange of value for money is done in a way that may be somewhat unique for a retail establishment, but isn't anywhere close to anti-capitalistic.
I thought that the existence of an individual or group of individuals who hold material wealth and can invest it (those who hold capital, or capitalists) instead of selling labor was capitalism at its core, and maybe therefore at its finest - is that not your take on it?