Can you sell an un-panted invention to a large company?

Your first answer from the Orifice seems almost fatally ignorant.

The vast majority of inventions in the world today are not patented anywhere. People still somehow manage to do business without waiting the three years or more to get a patent issued and without spending an ungodly amount of money on the lawyers and fees for patents on things that the competition started copying the minute your patent application was published.

Valuable NEW ideas and resulting inventions are often exchanged with others as "trade secrets", using contracts of various kinds, including "non-disclosure" agreements. In some cases, they might be more concerned about exclusivity and "non-competition", if you're also selling the same ideas to others or thinking about developing it yourself. Your contract would obviously include terms about sub-licensing to others (on to another company) and their on-going responsibility to protect it and pay you the royalties.

It is also false that "you cannot legally protect an invention which is not patented" in at least two ways: (1) inventions that are trade secrets are, by definition, protected but not patented (patents are public); and (2) you cannot protect an invention that *IS* patented, other than attempting to enforce your right to exclude others from making, using, selling or importing it in that country.

You should look into online articles about the concept of "unsolicited commercial ideas" and how to protect yourself from people who would try to side-step your rights.

Absolutely not. A company is going to need you to have, at least, a patent pending status.

Worst case scenario, they could actually just steal the invention. You mention a contract, but you cannot legally protect an invention which is not patented.

Even if you managed to get some sort of contract in place (which you won't) it would only be between you and that company. No other company have agreed not to steal it. There's nothing stopping them getting on to another company, giving away the idea and just profit sharing.

Get a patent.

they could make something that would not "study on" the claims of your patent. in the journey that your claims are not wide, then they'd probably paintings around them. besides, they could make your invention precisely and help you sue them. Suing them is rather costly.