Complex sentence: The paradox arises when a firm sells a product with only few substitutes, which in turn will increase the price of that product up to a point where the product begins to attract more and more substitutes.
Split into simpler sentences: The paradox arises when a firm sells a product with only few substitutes, which in turn will increase the price of that product. The original reason was that the price would continue to increase until a point where the product begins to attract more and more substitutes.