The phrase "free preview" means getting a chance to see or experience something for no cost before deciding if you want to purchase or commit to it. Full definition
The site does thankfully offer free previews of the volumes but even then they seem more useful for those considering purchase of the books. (kuriousity.ca)
On this website the entirety of volume one of One Piece will also be released for free preview over a number of weeks. (kuriousity.ca)
New titles and volumes, along with free previews, will be added frequently. (cbr.com)