Yes of course you can buy and sell ingredients any day of the game. For buying, depends what's available. For selling, you can sell what you have.
A simple way to make money will be to use the gather ingredients option and then resell them at the various vendors.
The new vendor system is much better than the one used in Loren: you can "mark" items to sell and then click "complete transaction".
For example, you have 100 gold. You check to sell 5 items for 50 total. But you see some more items you like worth 25. You don't need to first sell, then buy. You can just mark the ones you want to buy, then the ones you want to sell, and click "complete".
So at least the process of buying/selling will be less painful

