If you sell and buy around the same time, the market situation (hot, cold, before, after, or no crash) is of little relevance - it just affects the size of the numbers on both contracts, and about the same, so it's mostly a wash.
If you want to take advantage of your prediction, you have to split the selling and the buying in time - several months at least - and sell high, wait until after the predicted crash, and then buy low. Obviously, that implies that you rent in between (or live in your car, or with your parents, or whatever). And if the crash doesn't come - because your prediction was wrong - you don't make money on it, but lose the value gain from the period you waited. No risk, no gain.