Final answer:
The Internet is the factor that has most significantly transformed buying and selling across industries, leading to increased competition due to the rise of business-to-business platforms and globalization.
Step-by-step explanation:
The one factor that has most significantly impacted the nature and core of buying and selling in nearly all industries has been the Internet. The emergence of the Internet and advancements in communications technologies have revolutionized markets by enabling consumers to purchase goods and services from anywhere in the globe. As a result, local retailers now face increased competition not only locally but also globally. This effect is even more pronounced in business-to-business markets, where business-to-business websites facilitate connections between buyers and suppliers worldwide. Furthermore, the phenomenon of globalization is intertwined with technology, leading to a more interconnected world and intensifying international trade and financial flows.