I suggest getting him checked for a micro-chip. I wouldn't take him a shelter, that's for sure. Unless it's a no-kill shelter.
I believe that if you find a lost dog or cat, the owners are more often than not irresponsible. A responsible owner makes sure that their animal is kept in a safe area, and contained properly.
Though I do understand that accidents happen, it's usually safe to assume that the owners either let them out on purpose because they're stupid or the dog keeps getting out but they won't fix the problem.
I think that's a poor assumption to make.
In 2008 my dog went missing. My husband and I went on a 4 day vacation and left her in the care of his father. The day before we were supposed to come home she lunged on the leash and broke it. She was missing for 6 days... the worst six days of my life. Does that make me irresponsible? I owned her for 5-6 years at that point and she'd never gotten out of the yard or even gone outside without supervision under
my watch and because the leash failed she got lost.
Some circumstances are beyond a person's control, it doesn't make them a bad person or a careless owner. :shrugs:
When my dog was missing I did the following: I posted signs everywhere, dropped them off at Vet offices, left them at the pound (and went to the pound daily to see if they'd picked her up). I didn't check craigslist or use any online tools. I did everything locally...
So I'd suggest making a Found poster and giving a copy of it to all the vets in your area and in the area where you found the dog. I'd give one to the shelters, too. You might even post it around on some telephone poles in the area where you found the dog as well. I think it's great you put something up on cragslist, but what if the dog's owners don't have internet access?
Just some thoughts I had. I never would have found my dog if I hadn't made signs and taken the time to spread them around. Several people called to tell us they had seen her and the day we found her it was because a woman called and told us she had just seen her.
Oh, one last thought. You might also try posting signs at the post office. I found a dog around where I'm living at now (some years ago though) and that's how the owners found me. They'd gone to get their mail and saw the found poster. Also grocery stores and gas stations might be good places to hang found posters.
Good luck, I hope you find the dog's owners.