There are no specific or general rules when you or we should buy and sell but there are some tips:
-Buy when it dip or dump
-Sell when price goes up

Note:
Trading are risky as you need to monitor the market and the xoin you want to buy. Better to check the history of the coin before buying it so you will know if this coin are been pump and dump out.
Check the suppy of it
Check the dev. Abd back up company.
Have focus on what you do and of patient and understanding.
Happy trading...