Lens

Demon Eyes drift in all night and the stack grows faster than you plan for, so the surplus question comes up early. Vendoring them is the worst answer. Turn them into a Suspicious Looking Eye instead, summon the Eye of Cthulhu, and the boss drops alone are worth more than the raw material ever was. You also get the progression out of it, which you were going to need anyway.

The same trick scales up later with mechanical summons, but the payoff gets worse rather than better. The Twins fight back properly, their drops need processing before they are worth anything, and a failed attempt costs you real time. Farm the Eye while it is still a pushover.

Archery Potions burn through them, and the Mechanical Eye, the Deer Thing and the Optic Staff further down the line all want lenses in one form or another. Running dry means another night standing on the surface waiting for eyes to float past, which is the least interesting way to spend a night. A stack in a chest costs you nothing.

One small oddity: the Merchant brags about selling lenses in his idle chatter and has never once stocked them.