The term "Bull Market" is used to describe when a market is going up. Therefore, the term "Bear Market" is used to describe when the market is going down. Therefore, you'd be better off not to sell in a bull market and not to buy in a bear market.
Nevertheless, there are some traders that say they like to trade the retracements. This is like trading against the trend or trying to swim up stream. Yes it can be done, but why trade the hard way for the least amount of money? Instead, you can trade with the trend or with the current and make more money with less effort.
If you sell when the market is going up or buy when the market is going down you will make less money with a greater risk of loss. I have heard people say that they like to do dollar cost averaging and they keep buying when the market is going down, with the hopes that the market will turn and come back in their favor. Some times it does come back and other times they get a margin call before the market turns and comes back for them.
It is smart trading to learn what the market is willing to give and then taking it. Do not try to get more than the market is willing to give or you will loose some or all of what you have. You are getting in tune with the market when you want what the market wants and take what the market is willing to give. Nothing more, nothing less.
The terms "Bull" and "Bear" come from the manner in which each animal attacks. The bull uses its horns and thrusts them up in the air, whereas the bear swipes down with its paws. So, if the trend is up, its a bull market, if the trend is down, its a bear market.