Buying second-hand phones is hit-and-miss, not my thing anymore. That said, there are stalls at MBK that have them.
I was over there yesterday, and I'd say at least half by my reckoning of the stalls and venders that used to be on MBK's third floor are gone. Market forces at work, with the covid and slowdown, lack of tourist customers, etc.
If you can bite the bullet and free yourself of iphones, there are a bevy of android phones with impressive specs in the 3000-4500 baht range. Truly amazing what one can get these days.
There is a shop at MBK definitely w