I probably couldn't trade my cow for magic beans. I'd feel too sorry for the cow. I mean, say the cow and I go waaay back. Maybe she taught me to swim.
I could try to swipe the beans and run, but the cow would probably slow me down, and we would both be caught.
And it's not like I can take the cow home and come back alone to steal the beans, because how often does one ever really run into the magic bean guy?
So I think I'd have to very quickly grow the beans, climb the beanstalk into the clouds, take treasures, and run back to buy the cow again. If the cow has already been resold and I can't find her, I hurl myself from the top of the beanstalk. Nobody wins.
By the way, I hate beans. If I'm handing over all my worldly possessions for beans said to be magic, they had better be sparkly and colorful. Then again, the real magic beans will likely very normal-looking, just to mess with my head.