Anything that you have bought with Taiyaki can be sold on any stage and you will earn more gold in the stage (which results in more Taiyaki at the end of the stage). This is not a permanent loss, so you can do it on next stage again and again.

For example, if you are able to beat a stage without Miko's weapon (only using 35P's power), then you can sell all Miko's weapon and earn extra gold. These extra gold becomes lots of Taiyaki when you win. Then, you can buy more stuff with the Taiyaki, so that you can sell even more stuffs for gold in the stage. Rinse and repeat.

If you can do this on stage with more days (e.g 5 days), then you can earn more Taiyaki for less gold (like 1 Taiyaki per 50 golds instead of 200)

Note: If you want to earn the Taiyaki back for something else, you can always sell the stuff in the main menu.