The only option I see on bittrex is btc to zec. I do not show zec-btc therefore is it true I cannot trade zec-btc there?
of course you can sell, your Zcash in bittrex ..
just go to your account on bittrex
then , you must sent your ZEC to Bittrex
after your ZEC is show in bittrex wallets ,, you can sell your ZEC to BTC