Obviously this is just for new and casual buyers.
If you purchase a coin or basket of coins - and your stack goes into the red..
Do NOT panic sell. I know this seems like common sense, but the amount of people that do it is shocking.
The market is now old enough to show us that just about any coin can rebound. And in the long term can climb significantly.
When you sell in the red you are already down. You then have to live with the high possibility of watching your coin potentially increase significantly over the next weeks, months, years
So if you do sell in the red, be prepared for that eventuality. This is coming from someone who has seen people sell 1,000 Monero for peanuts, hundreds of Ethereum before it exploded, etc, etc. Personally I bought LTC during the 2013 spike, watched it decrease for 4 years, then turn around and make a 300% profit
Selling is completely up to the buyer - but be prepared to be able to live with fact that the coin you sold may explode at a later date
I'm not a trader, I don't play the markets - I hold for long term future value and so far any holder I know to date has made a significant stack over anyone else trying to repeatedly buy/sell in the market (excluding good traders)