A seller with one item has one shot at being found. A seller with four has four, and the fourth costs far less than the first if the shared parts were designed to be shared.
Where the reuse actually is
Authentication, roles, settings, the admin shell, the mail layer, the installer. None of that is what your item is about, and all of it is what took the time. Get it into a core you can carry forward and the next item is mostly domain work.
Aim each one at a different buyer
The point is not to sell the same thing twice with a new logo. Buyers see through that quickly and rate it accordingly. A booking core can become salon booking, clinic appointments and equipment hire, each with its own fields, workflow and vocabulary. Those are three different products to three different people, however much code they share.
Keep the core private
Ship it inside each item rather than as a shared dependency your buyers have to install. They bought one thing and should install one thing.
Version them independently
A core fix that goes out to all four is fine. A version number shared across all four is not: buyers should never see their item jump from 1.2 to 2.0 because something they do not own changed.
The trap to avoid
Do not generalise the core so far that each item becomes a thin configuration of it. What buyers pay for is the decisions you made for their specific case, and a core that refuses to make decisions pushes all of them back onto the buyer.
