Re: who are those buying when the price is down and almost everyone is dumping ?
If the price falls, investors who do not sell coins must invest in bitcoin. I recommend selling other coins and holding coins is a high risk. Unless you are able to judge the short-term cryptocurrency market, selling coins is the best way.
I understand you are trying to mean people who do not prefer selling even the market is falling down must go for investing only with bitcoin and should not touch any other coin, right ? It is always high risk involved if you are going to hold any other altcoins for longer period because anything may happen with them and we cannot be sure about their future unlike how we are being confident about the futures of bitcoins. We must be having different plans for bitcoin and altcoins basically.People(investors) have their own strategy and i believe that for them to buy during the deep they have already mapped out their strategy
Yes, having unique trading strategy and following them strictly is more important to crack profits from any markets. Usually I'm not buying when everyone is dumping but I will go for buying bitcoin when it will be available comparatively cheaper. I mean to say I will not buy when others are selling but I will go for buying when others just finished off their selling 
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