I used that DJM-300 from probably 99-2014. It was an absolute tank. Aside from two crossfader replacements, it didn't have any issues. If the RCA's didn't get smashed in as result of a drop, I'd probably still use it.
Since then, it seems like anything under $1000 is going to have extremely poor quality potentiometers and channel faders. Even with big names like Pioneer, their low end mixers are almost guaranteed to have scratchy pots and faders in under a year. And, brands like Numark will have scratchy pots in two months.
Which, is hard for me to understand. I built two mixers for my eurorack modular setup probably 5 years ago, and used alpha pots (which are considered cheap, and cost $1/piece). No scratchy pots at all; and they get considerably more usage than my DJ setup.
I've been passively looking for something to diy, but none of the offerings are aimed at a 3-band EQ DJ setup. I'll probably break open my current mixer and replace all the pots, but it's sad that I need to do such a thing. Especially considering the retail cost of parts will be $10.