I really don't even know what to tell you....those people that had that poor little thing at a flea market should have their flipping head examined!!!!! That puppy should be with it's mommy until 10-12 weeks old. But that's neither here nor there, that is a moot point.
You probably are getting a lot of good advice from this site, and I would highly recommend contacting your vet as others suggested, ASAP!! It was a good idea also that someone suggested calling a breeder.
I wouldn't think a puppy that age could have any solid food, but sorry I'm not of any help because I never had a puppy that young.
Good luck... Please keep us posted.