Sayyiduna Abu Hurairah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) forbade deceptive sales and the sale of Hasat (pebble). Imam Darimi (may Allah have mercy on him) said: Bay' al-Hasat is that when the pebble is thrown, the sale is completed.
(Explanation of Hadith 2598) One form of «بِيْعِالْحِصَاةِ» is that a person throws a pebble, and whatever item the pebble lands on, its sale is concluded. This type of sale was prevalent in the Age of Ignorance (Jahiliyyah). And «بِيْعِالْغَرَرِ» is when there is uncertainty regarding the acquisition or non-acquisition of something, such as selling a fish in the river or a bird in the air. In both cases, there is deception and uncertainty, due to which this sale is unlawful and impermissible. There are other forms of this as well, which can be seen in Sharh Bulugh al-Maram by Shaykh Safiur Rahman.