After reaching a certain part of the story, the General Store will open up on the world map. Here you can buy and sell items you've acquired to Nitori. Items you sell are listed in her shop and can be bought back until you leave the store.
The Power Amplifier uses special items called strength stones to increase your stats, and each stat can be powered up 10 times. You can also increase the number of times you can restore health from vigor. Note that with each stat increase, the number of strength stones required also increases.
Getting Strength Stones
At Kogasa's Smithy, you can pay to have your weapons reforged to change their attributes. An item's stat change will occur randomly within a set range, and the results are shown after the reforge is complete.
Item Attribute Range
Reforge the Reforge
You can keep reforging an item so long as you have the money to pay for it. Keep trying until you get the stat boost you're looking for!