Can a private seller decide to keep their car after a verbal agreement was made and shook on?

The fact that you “shook” does not mean much. In California a contract for sale of goods over $5000 generally must be in writing absent certain...

Call around as suggested. Otherwise, we are attorneys, not auto repair experts. On top of that, your post does not, I am sure, contain the...

WE HAVE A TRAILER BUSINESS AND SOME DROPPED OFF THEIR TRAILER AND NOW WON'T PICK IT UP, WHAT CAN WE DO?

Turn the all caps off. I do not know what you mean by a “trailer business” because that can mean a lot of things. I do not know what you mean by...

You would need to hire an attorney to review your terms of service as well as the advertising for the bag. I doubt the three days complaint is...

We are not doctors. We have not examined you, we have not seen a lab test result of the product. We have not read the products warnings, etc.

Can i repo my truck back?

If you can get it without violence, get it. Hopefully you kept a spare key. If not you will have to have it towed.

We have not read your paperwork and as stated above it depends on what you previously agreed to. Discuss with a real estate attorney or your...

Probably. You need to have an attorney review everything. There may be a lot more going on here.

Well since you have listed exactly zero defenses you would have to such a lawsuit, why do you think you would have any chance of winning? Nor...

Can I protest an email by citing the contract i previously signed?

Probably the contract but your post is exceptionally vague and also none of us have read the contract.