What's the difference between Finder and Keeper?
What's the difference between the Merchant Crown's Finder and Keeper abilities? Also, how can I increase the success rate of these abilities?
According to Gamefaqs the keeper ability is just a more effective version of the finder ability.
While there doesn't seem to be a way to increase the success rate, there is a trick for point farming which can be combined with finder/keeper to farm gems too. By using two wayfarers and auto-battle you can immediately escape from any battle you enter and earn 5 points. If you have a merchant wearing Hermes Sandals set to use finder/keeper it will harvest a gem before you escape from battle. Go to somewhere with enemies, and tape down the d-pad so you're running into the wall. Come back later for free stuff. Source (Ctrl+F Point Harvesting Trick)