That's an offer that's hard to turn down, or at least to look into a little. And the sign is right by the house with the dog sat on the porch, so it's easy enough to check out.
"So, you can talk, huh?"
"Yep" says the dog.
The man is surprised and impressed. "How did you learn to talk?"
"Oh, I was part of a government program run by the CIA to create a new genus of super dog, genius level intellect and top quality skills. I was the only one of the litter who could talk, so they fast tracked me up to being an agent. They taught me how to use a gun with my mouth, how to drive with my paws, how to diffuse a bomb with my tail, and even how to parachute into hostile territory. Apparently, thanks to the genius genes, I learned even faster than the human agents, and I was sent to Russia as a spy within a month.
"I had an amazing time chatting up the local pooches, begging diplomats for treats, and feeding top secret info back to the states. My work even helped delay the war on Ukraine by 2 years, but I blew my cover when I called Putin a jerk straight to his face. He sent his top guys after me, and I had to flee through the city. Jumped onto a train through Serbia and swam across the Bering Strait into Alaska.
"Unfortunately, Putin sent an assassin after me as I was moving through Canada, and he caught my dominant paw. I had to take the guy out with one of my ears, but I managed to survive, and my wounds healed really well. Now I'm retired, so I decided to keep this town safe."
"Holy crap" says the man, transfixed. When the dog's owner comes out, he looks up at the guy and asks "Why the heck are you selling him for only $5?"
"Cause he's a frickin liar! He's never spent a day outside of Utah!"
An all time classic!