Two months after a market phenomenon took shares of GameStop to the moon, the video game retailer said Monday that it will sell up to 3.5 million of its shares. The shares will be sold through an “at-the-market” offering, which lets companies place their stock on the market over a period of time. The announcement sent shares of GameStop, up 850 percent this year, down 8 percent at the opening bell. The GameStop saga has been one of the biggest stories on Wall Street this year. The company had been pummeled as new technology allowed people to download games, rather than buying a physical copy from GameStop or somewhere else. That shift threatened the existence of GameStop and its shares had been more than halved, to $20 each, by the start of this year. A number of hedge funds, believing the value of GameStop shares would fall further, shorted the …