The best time to sell altcoins is when it hit your target price. You don't need to wait for moon as it may not come but when you have a target price in mind then you can easily let go of that coin.
Yeah, or when you strategy to sell is already playing out. Normally, we always have what we look for as a trader and most of the time, the best way to sell an altcoin is to have a specific target and stick to it without being too greedy. It is not easy to time a market but at least, you can make use of the movement in the market to do yourself some good since you have bought during accumulation phase and you are already making huge profit.
The best time to sell altcoin is when btc increases slowly. We will sell it and get it. I trade a lot and I see that!
If you are looking at when bitcoin increases slowly, then you may actually not be getting it right. The thing is that I do not day trade or do short term trading, but for trend following, just let bitcoin be in an uptrend and then be stable, then it makes it easy to look at altcoins and then buy into them, and as they keep going up, monitor their trend as well as that of btc, once bitcoin trend is changing or theirs is changing, that is the perfect time to get out of them.