You will unlock this simply as you play through the game. I unlocked it 20-30 hours into the game.
As you explore dungeons you'll pick up items that can only be used to sell to merchants, and you'll be able to sell equipment and items you pick up as treasure throughout.
There's no real way to rush this but generally just make sure you collect all the items possible and if your inventory is filling up try put as much as possible in spare slots on your equipped inventory.
At some point you'll probably find you have lots of potions etc you don't use, so the odd spring clean of unused potions can help stock up on gold, spare equipment slots and pushing up the items sold to merchants.
By the way - every town has a merchant, so you can sell stuff any time you're in a town.