Trade-in quotes vary far more than people expect for the same model in the same condition. Some of that is the market. Most of it is information — the parts a buyer cannot verify get priced as if the answer is the worst one.
Sell it sooner than feels natural:
The steepest drop happens in the weeks after a successor launches, not gradually over the year. A phone sold the month before the next model arrives is worth substantially more than the same phone sold two months later, in identical condition.
Clean it, and photograph it honestly:
Not to hide anything — the opposite. Clear photographs of the actual marks get a firmer offer than vague ones, because the buyer no longer has to price in what they cannot see. Front, back, screen switched on, and a close-up of any damage.
Know your battery health:
It takes thirty seconds to look up and it is one of the few numbers that moves a quote on its own. If it is low, say so; a replaced battery is worth more than a hidden one is worth pretending.
Find the box, and the network status:
The original box is worth a little. Whether the handset is network-approved is worth a lot — often the largest single factor after the model itself.
Then let more than one buyer see it:
A single quote is a number with nothing to compare it to. Requests here go to several shops at once, and you choose from what comes back rather than accepting the first offer made.