The reason anybody is willing to buy and sell anything is because there's no single "real value" of anything. Value is contextual. When the grocery store gives me a cake in exchange for $20, it's because the value of the cake, to them, is less than $20. Conversely, the value of the cake to me is more than $20, otherwise I wouldn't be buying it.

If you sell your business, it's because the value of the business to you is less than the purchase price. Likewise, the value to the buyer is greater than the purchase price.