what...
Translation : "I run worthless crapware via Termux on them, that ends up generating negative income once you factor in electricity cost".
You could actually run an actual legit miner on the thing, but yeah, you're not getting ahead your electricity usage.
You're assuming it isn't plugged in to someone else's power though.
The seller didn't said they pay for their electricity. If they live in a dorm, or charge the phones in their workplace or school, there are lot of illegal ways to get free electricity. It's easier to conceal a regular phone than an antminer
Btw I'm not buying the phones cause I know their batteries are fucked