I can answer the second part of your question, but not the first (I'm not an expert on the slave trade overall). First and foremost: yes, pirates did take human cargo for the purposes of selling those people into slavery. I am not aware of any secondary literature that tries to take account of the impact of piracy on the slave trade broadly, nor any that attempts to determine what percentage of pirate takings were slave ships. Nevertheless, there are historically prominent examples, such as Laffite's capture of a Spanish brig full of slaves in 1813 (the slaves were sold after capture). The Laffites were rather notoriously involved in the slave trade, and I'd suggest Davis' The Pirates Lafitte: The Treacherous World of the Corsairs of the Gulf (Harcourt, 2005) for more on them. So, yes there were cases in which slaver ships were capture by pirates and their human cargo sold, especially so in the later 18th and early 19th centuries.

Second, in 1820, the U.S. Congress actually passed legislation that made slaving piracy. The relevant section of the act makes it illegal for U.S. citizens or U.S. owned vessels to participate in the slave trade, regardless of the country in which the slaves were to be sold. If caught, the act says, "such citizen or person shall be adjudged a pirate; and on conviction thereof before the circuit court of the United States for the district wherein he shall be brought or found, shall suffer death." This act complimented the law passed in 1818 that overhauled the original 1807 ban on importing slaves into the U.S. But, in a very literal sense, the slave trade was piracy in the early 19th century.

This isn't directly related to your question, but it is something of a historical curiosity. In 1816 a ship called the Plattsburgh was pirated by her crew (via mutiny and murder) on a voyage from Baltimore to Smyrna (Izmir, Turkey). The pirates' capture, trial, and execution in Boston were wildly popular events throughout the United States in 1818 and 1819. Upon the ship's return from the pirates, the Plattsburgh's original owner sold it, and the new owners apparently outfitted it as a slaving vessel. In 1820 the Plattsburgh was captured, and its owner and captain were tried and convicted of slaving. Niles' Weekly Register reprinted a Boston Daily Advertiser article on the slaving case which notes that the ship was the same as the one pirated in 1816. You can read that article here, beginning on page 215 under "Slave Trade."