pump-and-dump

noun

the practice of buying shares, generating favourable publicity about them, especially on the internet, then selling them when the price accordingly rises

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012