There are multiple mentions that electronics ALLWAYS malfunctions in presence of magic.

I don’t think there’s actually any such mention. There’s several mentions about how “muggle technology doesn’t work in the Hogwarts grounds”, but there’s no mention of electronics going haywire when someone is doing magic outside of hogwarts, imo?

Please do correct me if I just remember wrong.

And even just turning out lights is something that is apparently not that simple to do. Aside from Peruvian instant darkness powder — which doesn’t exactly snuff out lights, but covers them in darkness — the only thing to affect lights is Dumbledore’s deluminator. And he’s a magical genius.

My point being even turning off the lights is challenging. Muggle tech may not work in Hogwarts but I don’t recall any mention of magic fucking up tech unless it’s magic specifically meant to fuck up tech. Hogwarts is just like such a protracted and magical place that “muggle tech doesn’t work” but even that’s kind of a silly overarching statement that’s easy to challenge. Plumbing is technically muggle tech. It works. Wouldn’t ball point pens work as well? I imagine those would be pretty highly valued commodities. I think muggle borns could easily flog biros for at least a galleon a piece. Which the muggleborn could then go and exchange for the value of the gold, getting probably hundreds of pound for a galleon.

Endless money glitch.

But yeah at least pens would work I’d argue. Something like calculators is easy to see being fucked by some ambient magic fields, but pens? Nah.