I entered Crypto market for 2 months and usually see the term "pump and dump", and some P&D groups. But i wonder they pump by selling and buying themself, so when price go up, who will buy it for them and they sell that for who???
With Blue-chip coins, someone could buy when it've pumped already because of good news. So with trash coins, who'll buy this pumped coin and why many PP&D group still live until now

You are asking a kinda pointless question.
There are many investors focused only on penny altcoins that are often pumped and dumped.
If there were no penny altcoin investors,the pump&dump method won`t work and no P&D groups would survive.Those investors don`t care about the "trash" altcoins.They care only about the short term profits.
Due to your post, my interest in finding a legit pump and dump group intensifies,

From the way your post sounds, I feel like, P and D could generate big profits. Come to think about it, why there are still a lot of people focusing on penny Alts?
1. Can hoard a lot of coins from a small amount of capital
2. Once its value spikes up, more investors will come in. Hence, another increase of its value
3. Easy to exit in just a short period of time
And I believe that this is doable even if you are just trading alone. Monitor the market or have yourself a working bot that could work where the coin is listed (i.e. Polo, Bittrex, Binance, etc.)