1) How do I prevent Bittrex from selling my altcoins automatically? I don't see the "auto-sell" option any more.
Go to setting > Click auto-sell on site setting > click show all balance and choose what coin do you want to disable auto sell and disable it. (
click this to see image, because its too large)
2) How do I buy altcoins again on Bittrex in the new interface? I don't see a way.
Click currency that you want to buy (ex BTC-BCC), scroll down and you will see buy and sell options.