Sell day lilies/fruit/chicks/rabbits/produce/goats/cats/dogs. Its not uncommon to see any and all of the above at the farmer's market section of the flea market.
Sell sprouted plants.
Hustle antiques. those old blue canning jars, those soda bottles from the 70s, any kind of memorabilia from sports, any kind of garden too/mechanic tool over 50 years old sell great!
Sell rag bags. Put 10 pieces of clothes your kid outgrew/you don't ever want to see them in like "Daisy Dukes". write the size, age, and sex on the bag and grab bag it off for 2$ a bag. unless it is leather goods, jeans, or boots, singling out is a waste of time!
Sharpening knives and tools. Did this one myself, all you need is the right tools and a power outlet. A decent, quality bench grinder you can bolt down, a hand grinder and a variety of discs, a pink metal preparation wheel and a fine wheel to remove pits and heavy scale, a buffing wheel, with coarse, medium and fine rouge, to bring it to a shop shine and a green wheel
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made to remove metal without heat buildup and at last, a black crayon. rub a layer of wax next to your grind area, if it begins to melt, back off and quench it., and then to make the hairs pop off your arm, a medium grit whetstone and an Arkansas oil stone in a jar of 20 Wt or baby oil Back in the day, I would do a small knife for a buck, a big knife for 3$ and a decent refurbish for 5$ and negotiate on axes, hatchets, machete's, etc, depending on how screwed up they were. Only a moron would do a sling blade for 5$ be sure to set up near the knife dealers as well, that can net you some huge cash quickly!
Shoe Shine Station. No shame in it, lots of us teenagers did it in the winter months when the lawn-mowing/yard-raking jobs dry up and there are no farmers around whose livestock needs fed.
Welding under the table. Ten bucks an hour or a dollar a minute, depending on the job.
Shade tree mechanic.
Backroom gunsmith/camouflage expert. I used to make a killing in deer season camouflaging deer rifles and bows for ten bucks each. I do USGI woodland, tiger stripe, snow job and timber rattler and flat-clear automotive hard coat over all for durability. for 5$ more I'd hit it with rubberized truck bed spray for that tactical, texture grip that was just becoming popular thirty years ago. three jobs and my paint was paid for, the rest was smoke and beer money!
Arts and crafts. Make brick-a-brack out of old wood scraps, beer glasses out of bottles, knives, make rag dolls stuffed with real hay, make hillbilly furniture out of tree trunk slices, my uncle made a pair of cedar thrones using just a chainsaw and got 800$ out of them.
Roadside/flea market BBQ/sell canned preserves and BBQ.
Bootleg pretty much anything! Feeling sneaky? Beer on Sunday, "dry" sporting events, Sodas and chips where the concession stand prices read like a steak house.
Well, there's my list, hope it helps.