so here's how it works. let's say you own 40 stock. you want to sell. if more people want that stock than you have available the price goes up. good for you. if very few or no one wants it, the price goes down. bad for you. my personal recomondation is to diversify.
you win some stocks and lose others. in agragate you'll come out even, in the short term however you can expect to gain 2-5% of your investment. the key is to try to predict what you think everyone else wants. if you think everyone is going to want stock A in two turns, you want stock A in one, and then sell it on turn three, unless you expect more people to invest turn three.
This quick selling strategy, though effective, is often risky and very selfish.