Which saying means to sell a part as the whole?

I'm looking for an English or foreign saying which mean that it is made to believe that part of a thing can achieve what the whole thing only can achieve.

Examples :

To sell a wheel as if it was a car. I don't know what a car is but I'm sold its concept (a mean to transport quickly) in the form of one of its part, the wheel. Only later I will realize that I can't do what was claimed without the other parts.
To sell weights as if they were a full gym.
To sell bricks as if they were a house.
To sell a spacesuit as if it was a spaceship.

It's the notion of benefit rather than quantity that is to be expressed. All creative sayings are welcome if they fit better.

The real case that brought this question is sellers of online courses (or books) who sell incomplete knowledge to achieve what is claimed. It is different from selling a miracle product.

Solution 1:

There are many sayings that contrast small with large for effect. We may discount most of them, such as a “a sprat to catch a mackerel”, “spoil the ship for a ha’porth of tar”, “thin end of the wedge” because they do not deliver what you need.

The first I can offer is “a pig in a poke”. The buyer is offered the poke (a bag), but does not really know what is in it (probably a pig, and of unknown quality). Even this saying is not a perfect fit to your specification because I have to argue that the poke, although encompassing the pig, is smaller (in weight or value) than the pig.

Dictionary of Scots Language

Poke:

A bag or small sack. In commercial use appar. sometimes as a more or less definite measure of a commodity, smaller than a sack

A second suggestion is “buying a pup”, or its corollary “selling a pup”, which fits your need. The buyer only gets a small pup when they really needed a grown dog.

FreeDictionary

buying a pup:

To buy something that ultimately proves to be worth nothing or less than promised.

Solution 2:

A word that conveys selling less than you promise is

shortchange (v.)

Deprive of or give less than something due: CHEAT m-w

To deprive someone of something for which they paid. Wiktionary

Treat unfairly by withholding something of value.

Frankly, to provide anything less than the above requirements is unconscionable, and as a digital camera maker you know better that to short-change your customers this way. Lexico

Chances are good that you'd feel shortchanged if you bought a book in which the first nine pages were blank. Bryan Peterson; Understanding Digital Photography (2011)

However, it could still sit on the dealer's yard for several months before it is actually sold to a customer who is then shortchanged on their warranty. "Buyers being shortchanged on warranty" at carsguide

The groups say some Chinese companies routinely shortchange their employees on wages, withhold health benefits and expose their workers to dangerous machinery and harmful chemicals, like lead, cadmium and mercury. David Barboza; "In Chinese Factories, Lost Fingers and Low Pay"; The New York Times