avoiding code redundancies for a calculation in Java

You can use a +1 or -1 coefficient based on isBuying value.

newPrice = Math.round((stock.getPrice() + (isBuying ? 1.0 : -1.0) * roundRandomNumber * 0.1 * amount) * 100) / 100.00;

You could try by extracting just the factor in the condition:

public void changePrice(Stock stock, int amount, boolean isBuying) {
    double roundRandomNumber = Math.round((0.5 + Math.random()) * 100) / 100.00;
    double newPrice;


    //calculates plus or minus depending on the buyoption
    double factor;
    if (isBuying) {
        factor = 1.0
    } else {
        factor = -1.0
    }

    newPrice = Math.round((stock.getPrice() + (factor * roundRandomNumber) * 0.1 * amount) * 100) / 100.00;
    if (newPrice > 0 && newPrice < 3000) {
        stock.setPrice(newPrice);
    }
}