Who or what determines stock market prices?
Market makers who are the middlemen that hold an inventory of the stock and post prices that they will buy and sell it at. They agree to buy it for one price and sell it for another price and make their profit on the difference. The prices are adjusted until the two parties can agree and the trade is done. Its a lot like any second hand dealer in a product, which is what the stock market is. The secondary market for stocks.