regrets after selling Bitcoin mostly happens because no one can perfectly predict the coin market.
Prices move up and down it's just based on supply demand and sentiment.if after buying,the price goes down immediately just wait untill it's goes up if you are not urgently in need of money at the time,you just can't predict this things.but it's always save to buy coins when the prices drop .
Your statement here is wrong, Being urgently in need of money should not be the determinant for selling bitcoin, people regret selling bitcoin because they sold early and because they chose to trade with bitcoin rather than invest in it which is a financial mistake, no one should be trading with bitcoin on any normal circumstance instead just keep accumulating with DCA, waiting for the DIP when you can DCA is also a financial mistake as it shows a tendency to sell immediately the price goes back up so don't be telling people it's okay to sell if they are urgently in need of money, this is why having an emergency fund in important as an investor so you don't ever have to sell too early.
First of all, anyone that feels like because he is urgently in need of money, his Bitcoin holdings is the only option , so he has to sell to have money, it's either that person is not investing with his discretionary income or he has no emergency or reserve funds in place that protects his Bitcoin investment, because if he is truly during the right thing, their is no way his Bitcoin holdings will be the first financial casualty just because he is in serious need of money.
So it's either the said fellow is not investing with his discretionary income or he did not put down measures to safeguard his investment like having an emergency or reserve funds in place for the rainy days.