Pigronata

A pinata you hang from a ceiling and hit until it bursts. Coins come out along with a small chance at party themed extras, and a fair share of them give nothing at all. Bought at face value and broken one at a time it loses money slowly, so treat it as a way to get a specific drop rather than as a coin farm.

Party Bullets are the reason to buy these in bulk. They are otherwise a Hardmode item, and grinding these is the only route to having them earlier. The Flask of Party is the gate: no flask in the drop, no bullets, though the flask can turn up on its own.

Hang them at the point where a grappling hook is anchored to the ceiling and each one breaks the instant it goes up. A Brick Layer and a Builder Potion tighten the place-and-pop loop enough to matter over a few hundred of them. Watch out for the Guide to Environmental Preservation, which makes them immune to everything except a pickaxe and quietly kills the whole method.

The margin flips if you drive the purchase price down. Housing the Party Girl somewhere she likes, next to neighbors she likes, with a discount accessory equipped, gets the cost low enough that selling everything that falls out comes back ahead. The gain per pinata is thin, so this is only worth setting up if you were farming them for the bullets anyway.