This is a right time to buy bitcoin but never a good time to sell. We may even borrow to buy so that when next the price goes up it will generate more profit. Also do not used all you have to buy at once because the price may still be going down day by day. It may work fine if you divide your money into different batch and buy bitcoin by batch weekly within this period of time.
We can't really say when it would be a good time to sell because people have different needs. With the price this high, it can even be argued that it's a good time to sell especially for those that jave kept their btcs since last year. As long as you feel good about it, its always a good time to sell.
Yes this makes sense. Everyone have different strategy of work. Everyone comes with some different needs and problems. So there are only two options for an individual, either to keep holding or sell it. So it is only you now, who have to decide according to the needs what to do. Like you said, it is best time for a person to sell who have kept bitcoins from last 2 3 years.