Of course it is, your coins aren't just disappear at all, they just transfer from this guy to other guy. In your case, when you're selling, there is one guy in the market has the purpose to purchase your coins, So the bitcoin is yours and your altcoins are becoming his coins. So choose the right time to buy or sell will make you become the smart trader.
Making a deal at the right time is the key to success in the crypto market especially if you are involved in trading. Smart traders are always making big profits because their timings of confirming deals are excellent.
Indeed the coins keep on circulating when we buy or sell them. This is what I really like about the digital currencies. They are not going to stay with a single person forever.