Which innovative pricing strategy first emerged around 2010 that significantly disrupted traditional mathematics textbook sales?

The mathematics textbook market has seen significant shifts in pricing strategies over the past decades. Publishers have had to adapt to digital disruption, open educational resources, and changing student expectations. This trivia question explores a key pricing innovation that dramatically changed how students access higher-level mathematics content.

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Subscription-based access to digital mathematics content libraries
Buy-back guarantees where publishers refund 50% of purchase price after course completion
Dynamic pricing where textbook costs decrease proportionally to class enrollment numbers
Peer-to-peer pricing where students set resale values through publisher-mediated platforms