Is there a law to protect vulnerble seniors on a bad car deal?

I got talked into trading for a 2008 super duty ford 4wd super cab there wasn't enough diesel fuel in it to drive it more than a few blocks, so I relied on there honesty how good the truck was & how much it was worth and how little my car was worth. The sales man told me the truck was worth 26000 with 138000 mi - I made the deal at 16,500 dollars and my 2003 mits eclipse spyder convertable in mint condition with 68000 mi. I put more fuel on the truck and drive it some more and after it warmed up it started to smoke profusely- make noise in the front 4wd axels.& transfer case. So I check on the blue book value and find that its worth no more 16,500 in the poor mechanical condition its in.---what can I do - I would like to return it and get my car and money back.

There's a law protecting those with diminished mental capacity, but I don't think that applies here. You failed to do your own due diligence. You took the salesman's word as to the value of the truck, made a deal and now you have to live with the consequences. There's no right to return a vehicle.

"Vulnerable"

And no, not unless you have documented mental issues, as mentioned in another answer.

You had ample time to do your due diligence, have the truck inspected by a licensed mechanic, etc. You had ample time to look at plenty of online sources to review vehicle values. Nobody held a gun to your head and made you sign paperwork.

The dealership may work with you on a return, but no law forces them to do so.

Nothing to stop you adding more fuel or getting the salesman to do that.
If they misrepresented the truck you may have a claim against a trader it depends on the laws where you are.
In the UK a trader has to represent a vehicle correctly and offer a waranty on it.

There basic tenant is buyer beware. I'm still trying to figure out how you went from a sports car to a heavy duty pickup truck.

Get real. You just paid some very expensive tuition.

Unfortunately, there are no such laws. Used cars are sold "as-is" which means the responsibility is on the buyer to make sure the vehicle is in good condition and priced right BEFORE the sale.