The best way I've seen for a side gig is something where you can really add value. I like flipping as opposed to reselling.
For example buy a cruddy coffee table or end table for $10 and refinish it really well and sell it for $200.
Buy a dirt bike that someone doesn't know how to maintain and rebuild the engine.
I like these side gigs more because you can work them at 3am, there's no customer time table to work to. You can't cut grass or clean out apartments at 3am.