You need to complete Lily's first request to have some start up money for this to work, and make sure someone is selling items at the bar. Preferably Lily or Electra, for the access to Ice Cream.
1) Buy all the milk you can.for 120g
2) Cook it into Hot Milk at the inn's kitchen.
3) Sell it back to the store for 200g.
4) Buy Ice Cream as needed to replenish RP.
Rinse and repeat until you are satisfied or bored.
Note: This can be somewhat game breaking, because on the very first day you can easily exploit this cheat to be able to afford all House Upgrades, any armor, weapon, crafting material, or medicine you want, and because the whole cheat takes place in the inn time will not pass while doing it.