Samsung Galaxy S10+ 512GB with the gun gray ceramic back.
I got it from a colleague after it had sat in his drawer unused for a year, because he switched to iPhones almost immediately after getting the S10+. I was shocked to see that the 512GB variant still sells for around €400-450 used here in Germany. So I was haggling with him over the phone for like six months but we couldn't ever agree on a price.
Then, through my job, I got a broken QNAP NAS from a client, that had the known Celeron bug that plagues NAS systems of all brands, and I managed to fix it by soldering in a €0,15 resistor.
So it happened that my colleague really wanted to buy a NAS... and, needless to say, I got the phone, which he wasn't willing to sell for less than €400, for the price of a resistor and half an hour of research and soldering efforts. Good deal