As an antique dealer for over 10 years now; we hit hundreds of sales every month...it is rude to think other people will not mind your dog. It isn't just about the person giving the sale, but everyone else around who now has to watch out for the dog, as well as not knowing what type of temperament the dog has.
This also goes for dogs and children at estate sales. Neither one is welcome.
Thinking you can ask the owner is ok, is NOT fair to the owner..you are putting that person in an uncomfortable position. They are doing this to sell things..they can not speak for everyone else at the sale..when honestly, these types of places are not meant for pets! And if people do not think that people do not purposely bring their dogs to sales, they have no idea how large the garage sale culture is.
I have seen people bring their dogs to antique fairs and watched these tiny dogs pee on every table leg they pass by...the owner is completely oblivious to it. I watched one time as the owner told the dog owner about the pee and the woman ranted on and on about how good her dog was and would never do that. I have seen people bring large dogs who wag their tails and knock things off tables or taken a dump right in the walkways. I have seen dogs get into fights with other dogs.
I have seen people walk around with strollers with their pets inside taking up floor space from buyers who are trying to get around tables, only to not be able to get in due to the dog in a buggy.
I have seen children break things at garage sales, estate sales, antique fairs..and the parents just make excuses...
A dog is not human, a dog is an animal with animal instincts. If you want to take your dog out, go to a dog park. If you want to take your kids out, go to a child friendly environment.
It does not matter how much you love your dog...or your child.....this isn't about you. You are in a public place around other people...we constantly complain that if people thought about others first the world would be different..well, here you go.