You are perfectly entitled to accept, or deny a potential buyer. And from what you have written here, this wasn't anybody I'd have been happy to let my dog go to.
Is your 'crisis' so bad that you HAVE to rehome this puppy? In any case, it's always best to contact the breeder (most have return clauses in their Sale Contract) and hopefully they will take your puppy back, perhaps already having a new home waiting. Alternatively instead of using Craigslist, which will produce as many dodgy buyers, as people wanting to home their dogs, why not sign him over to a Shelter where, if he is a puppy still, he'll be found a good CHECKED home fast. Further, if he's 'a breed', most have Rescues with people waiting. And they too will check before placing.