The plan being, if someone attacks you, you run away and pour out the marbles behind you?
During a acorn bloom there were so many on the forest ground that it was easy to slip on them in sections, change forest ground to hard surface and acorns to marbles or ball bearings, and I can see that is could work.
Now if it is possible, which I say yes due to the acorns, to cause a fall, the other end of the question would be is the number, size,weight of the marbles/BB’s you need reasonable to carry and deploy. I would guess that depends on the circumstance.
It would certainly work if you happened to be chased while in a cartoon.
If I were in a cartoon, I’d carry a tin of paint and paint a tunnel.
No, you should paint a hole for them to fall into. 
And a bag of marbles would be great to beat someone with. But to run away? Wouldn’t they just go around?
A box of tacks?
But it’s not certain that you could beat him with your bag of marbles. What would you play for starters? Ringer, chasies, dropsies?
Caltrops. Pointy side always up.
If you are using marbles as your defense you better plan to always be on a hard flat surface or they will just squish into the ground or roll away. And hope your attacker doesn’t shuffle his feet.
Last year I learned to tie a monkey fist knot. I made one with a 1 1/2 inch ball bearing in the center.
Turns out they are illegal in a few states, similar to the restrictions on switchblades!
A pillowcase with unopened soda pop cans works pretty good too.
If humans can’t have monkey fists…
Don’t be silly, how can you paint a hole? 
At the very least, it’ll delay them.
But caltrops will remain dangerous to everyone who passes by, that could be considered as a weapon.
Hmm if I could find large steel ball bearings, I’d use them to burn holes in paper.
Search for ball bearings on Amazon. They’re reasonably priced and lots of fun. Buy some para cord while you’re at it.
i use monkey style self defense.
i carry a bunch of bananas with me. peel and throw.
If you have two bags, you can throw them in the air and your attacker will run away.
They’re about $20 each? http://www.amazon.com/Diameter-Chrome-Bearing-Bearings-VXB/dp/B002IFV8QA/ref=sr_1_1?ie=UTF8&qid=1361720458&sr=8-1&keywords=ball+bearing+large
Funny you should mention paracord, I just bought 30 m today. I just finished replacing my shoe laces with it, now I can carry paracord wherever I go!
I paid much less for 1 1/2 inch. There are different grades. I got a pack of ten for around $15.
Ball bearings in a leather bag is a classic blackjack or sap. Very, very effective close range weapon.
I’d really hate getting hit in the face with another mans ball sack. 
Wait! let me rephrase that. 