There is a Daily Training Quest for Lucre, but it's only limited to 3 times per day, and the limit is also shared with the getting Experience Tomes and Memory Gems.
It's recommended to always use up the 3 daily tries since the attempts can't be stored.
An alternative that's not limited to 3 times per day are the other item dungeons. Echoing Maze for Echoes is the recommended one since even the lowest rarity drop gives 600 Lucre, while the elemental items for unlocking Mana Board don't sell as much.
If you want to sell your items, go to the menu and select "Items" and select the "Upgrades" tab. Here, you can tap on any of the items to view the details or sell them for Lucre.
Out of all the currently available items, Echoes are the most expensive ones, and they aren't more difficult to farm compared to the other items. Even the lowest rarity Echo, which is the seed costs 600 Lucre per item, Echo Sprout is 1200, while Echo Bloom is 1800.
Masterful Merchant is a Memory Gem that gives a bonus to Lucre received after a battle. You can increase the amount of bonus that you get by Unleashing it if you have multiple copies. Leveling up the Memory Gem only increases the stats, not the bonus.