The most expensive mistake in selling a phone is not accepting a low offer. It is waiting, on the vague assumption that there is no rush, through the exact events that reprice your phone downward.
Those events are on a calendar and they are not a secret.
Samsung launches twice a year, so there are two steps
The S series lands early in the year and the foldables land mid-year. Each one repositions everything below it, which means a Galaxy owner has two repricing events to think about rather than one.
The practical effect is that the window between launches is short. A phone you were vaguely intending to sell at some point tends to sit through at least one of them, and that sitting is what costs money.
The two biggest drops, in order
The largest single drop for any Galaxy is around the announcement of its direct successor. Not the release, the announcement: the market reprices on the news, not on availability.
The second largest is the point where it becomes two generations old. That is the transition from being the phone people want but cannot afford new, to being the phone people settle for, and demand changes character rather than just softening.
Between those two events, value drifts down gently. That drift is the least urgent part of the picture and it is the part people worry about most.
The worst possible order of operations
Buy the new phone first, then get around to selling the old one whenever. This is what almost everybody does and it is the most expensive version available.
The old phone sits unused for the weeks or months it takes to get around to it, losing value the whole time, and frequently sits through a launch event while it waits. Meanwhile the money it was worth was not available to put toward the new phone, which is the one moment it would actually have been useful.
Selling first, or at least getting the offer first, costs nothing and changes the arithmetic on the purchase.
When waiting is genuinely fine
If the phone is still your daily phone, keep using it. Nothing in this guide argues for selling a phone you need, and the value lost by using it for another year is smaller than the cost of replacing it a year early.
If the phone is already six or seven years old, the steep part of the curve is behind it. The number is small, it is going to stay roughly small, and there is no urgency beyond the fact that it is not earning anything in a drawer.
The urgency applies to the phones in the middle: one to four years old, not being used, and still worth a meaningful amount today.
The short version
Galaxy value falls in steps around Samsung's two annual launches rather than declining smoothly. The biggest drops are at the successor's announcement and at the two-generations-old mark. If the phone is still in daily use, keep it. If it is not, and it is under about four years old, the cost of waiting is real and it is on a schedule.