Old DJM 400 – there is your problem right there
The DJM400 had most of its connections not really well isolated and the print of the boards were also pretty open and close to other circuits. Hence it is prone to get crackling during usage especially from faders and knobs.
Some of it can sometimes be mitigated by WD40 spray and patient disassembly and cleaning, but the time comes where faders and knob connectors are just too corroded to be any good. To replace those however is most of the time more expensive than the used mixer itself, hence why they are going so cheap.