A marketplace already has shoppers browsing it. You trade some control and branding for a chance at organic discovery you didn't have to build yourself.
A customer who buys from your own store is yours to email, remarket to, and build a relationship with — not a marketplace's user who happened to buy from you once.
A store with both its own branded page and marketplace exposure gets a chance at discovery while still building a direct customer relationship over time.
A Buylib store is fully yours — your own branding, your own domain-style store page, your own customer list. It can also appear in Buylib's shared marketplace for extra discovery, without giving up ownership of the relationship with anyone who buys from you.