To sell altcoins? Well then if that is listed in Binance, then you can consider using them. Safest might not be the right term but it's worth to play with the risks here providing how reputable that exchange today.

But if your alts is not listed there, then you can asked community first or do some self- research about all the exchanges where it was listed.

One good stop to check where a coin is listed is thru COINMARKETCAP. Just hit the "Market" tab after searching on a specific altcoin.