Don’t be stupid, any Giant with the forethought to ward against wizards would surely have wards against magic beanstalks as well.
(i have no memory of posting to this thread… seems a wizard has indeed been at work)
The cloud at the top of the beanstalk hides the island of Laputa (Jonathan Swift, Gulliver’s Travels), and the goose laying golden eggs is explained by Isaac Asimov in Pate de Foie Gras.
As an aside, I recall reading a sci-fi book that took the position that the story of Jack was a recounting in terms relevant to pre-technical peasants of an encounter with several humanoid races in conflict.
As I recall it (LONG time ago)
The “seeds” were (nanotech?) anchor points - planted in Jack’s field because it was the right place on the right world - which created an antenna tower to open a hyperspatial gate. This opened to the stronghold of the Giant (member of another humanoid race). Jack is drafted into participating. I think the essential mission was rescue of a kidnapped “woman” who was a musician who played an instrument by direct neural connection - the “singing harp.”
General searches and a search via Barnes and Noble fail to turn up the book, which I recall as being titled “Beanstalk.” Anybody else recall this one? 
Y’all are over-thinking it, even back 5 years ago.
Never attribute to malice what can be explained by stupidity. The beanseller himself was duped. He traded something good (maybe another cow?) for “magic” beans. Then later he figured he had been duped (maybe his wife hit him on the head with a rolling pin, to knock some sense into him.). So he did what anyone would do - suckered someone else.
Unknown to him, they really were magic (no quotes) beans.
Ultimately the story is about luck. In this case B is actually a Djinn who decides to mess with Jack by giving him magic beans.
Alternately the magic beans were drugs which Jack ate causing him to hallucinate.
I’m not nearly as clever as y’all. So I’ll use this:
“And then Jack chopped down what was the world’s last beanstalk, adding murder and ecological terrorism to the theft, enticement, and trespass charges already mentioned, and all the giant’s children didn’t have a daddy anymore. But he got away with it and lived happily ever after, without so much as a guilty twinge about what he had done…which proves that you can be excused for just about anything if you are a hero, because no one asks inconvenient questions.”
― Terry Pratchett, Hogfather
Then there’s the “Into The Woods” version.
The bean seller is a baker whose family has a curse placed on it to produce no children, because the baker’s father stole vegetables from his neighbor’s garden, the neighbor being a witch. In order to lift the curse, the baker must obtain four specific items to match a spell. One of the items is “a cow as white as milk.” Jack, a simple boy, owns a cow named Milky White, who is solid white, but Milky White is old and no longer produces milk. Jack’s mother sends him to sell the cow for much more that it is worth and not mention how old it is or that it doesn’t produce milk, whereupon Jack runs into the baker. The baker, not having any money on him, does happen to have five beans that were given to him by a stranger, that is really his father whom he never knew. The beans are from the witch’s garden, and thus magical, but the baker doesn’t know that. However, he needs Milky White, and so he coincidentally tells Jack the beans are magic, like any con man. Thus, the baker tries to con Jack but his lie is actually true, and Jack (following his mother’s orders) tries to con the baker, but the baker doesn’t need a healthy cow or a cow that has milk, it just has to be white. Therefore, they both actually got what they thought they were getting.
Then lots of other events unfold.
In this version, Jack steals the giant’s golden-egg laying chicken for his mother, then returns and steals the singing harp. The giant tries to retrieve his property, whereupon he falls to his death and smashes Jack’s house.
The giant’s wife then comes searching for the lad that killed her husband, and many deaths ensue. Plus lots of destruction. Finally, Jack, the baker, Cinderella, and Red Riding Hood (yes, their stories are part of the tale) cooperate and pull off a plan that kills the giant’ wife, saving Jack.
It’s not a happily ever after kind of story.
You need to get this movie: Jack and the Beanstalk: The Real Story.
“Jack Robinson is a wealthy business man with no time for anything but work. However, a family curse is looming over him - no man in the Robinson line ever lives past the age of thirty. With his upcoming birthday appears the remains of literally giant skeleton and a mysterious woman who claims to have once known the giant. Jack decides to go with her to another world where all is revealed to him along with the story of his ancestor, the original Jack and the Beanstalk. In order to save his own life and the world of the giants, Jack must right the wrongs of the past and return the magical harp and goose that lays the golden eggs to their rightful home.”
In the movie they show Jack actually taking 3 days to climb all the way up (he eats beans and drinks water found in the leaves).
Found it! It was “Beanstalk” by John Rackham. I found it by recalling the cover art - Kelly Freas - and searching the cover image. 
In Kate and the Beanstalk (by the author of the Magic Treehouse series, for those of you with kids) the guy who sold the beans was actually a fairy godmother in disguise. The giant had killed Kate’s father and stole his hen, coins, and singing harp. It was all a test to see if she was worthy of the inheritance. Could this be close to the original story?
Wasn’t there a version somewhere that the beanseller and the fairy (who guided Jack at the top of the beanstalk) were both friends of Jack’s father? And the giant had actually murdered Jack’s father and seized his wealth, and that the beanseller and the fairy were using this roundabout route to help Jack get back his own property?