Mostly small electronics repair & refurb.
I still can't believe that people are willing to pay me $25 to swap a $10 battery in a Nexus 5 - but I've done dozens of these. Though, to be fair, they're paying for my expertise at that point - I can do the swap, zero broken snaps, in 5 minutes.
... need to order more batteries, now that I think about it.
But I do a lot of small electronics repair, and a non-trivial amount of people will give me broken phones/laptops/etc for free.
Also selling stuff for people on eBay for 20% of the profits. Takes me 15 minutes for small stuff (I take good photos), often nets me $50+.
And electric bike repair lately. Amazing bikes, obtained for cheap-to-free, because they've got bad batteries.
Basically, I'm willing to do fiddly work that most people can't or won't do. :)