I notice that when there the price is going up so many people are joining the train and they also buy because the price is going up and they are going to make a profit, why people are not buying when it is down when the price will eventually go up and they can make a lot of profit.
The reason why most don't buy when the price is going down is that, they don't know if it will go deeper than their buy price and if it does, will it later go higher than their buy price and give them profit. There are most coins one can buy and it will keep dumping that the buyer might be left with no other choice than to sell at loss.
On the other hand, the reason people buy when the price is going up, even though it's risky is for them to make profit, but this is very risky as the aim might not be achieved.
However, from the two different cases, it all depends on the type of coin. Bitcoin for example dumped during the early days of the pandemic, most people bought and it later went up. Once in a while when the market is favourable, people buy Bitcoin or Ethereum just to take their own profit.
Therefore, the right idea is to buy good coins (Bitcoin etc) when they dump and when the price goes up, you sell and take your profit, if you are not a holder. Lastly, not all coins are worth buying when they go down at the same time, not all coins are worth buying when they go up.