Here is a straightforward look at this topic for Malachain.
A used phone can halve the cost of a flagship, as long as a few checks are made before money changes hands. Ask for the serial number and check it against the blocked-phone register, confirm that the previous owner has signed out of their account so the activation lock is off, and look at the battery health figure in the settings.
Meet in a public place or buy from a seller with a returns policy. Test the camera, the speaker, the charging port and the screen edges, and make a call. Factory-reset the phone in front of the seller. Then set it up fresh, update the software and add a strong screen lock. The whole check takes fifteen minutes and avoids the two most common frauds.
There is more to learn on this subject, and the linked website goes into greater depth. Treat this article as an introduction rather than the final word, and check any important decision against up-to-date, reliable sources before relying on it.
If you try any of these ideas, start small and adjust them to your own situation. What works for one person will not suit everyone, and it is fine to keep only the parts that help. Writing down what you changed, and what happened, makes it much easier to see after a few weeks whether the effort was worthwhile.
The most useful step is usually the first one. Pick a single suggestion, try it for a couple of weeks, and then decide whether it is worth keeping. Small, steady improvements last longer than ambitious plans that are hard to maintain once everyday life gets busy again.