Depending on the age of the dog, you may have to work with them on some social issues... but every dog should be trained.

I was (suckered) talked into adopting a little Blue Healer puppy that had been returned to the shelter TWICE. She was the sweetest most loving dog I've owned. She also ate two couches and an entire indoor ficus tree... but after moving that energy into fetching/Frisby catching she was a great family member.

Another one of the dogs I adopted had been shot before he worked his way into my home. Nobody knew about it until we had him xrayed after he ate a sock... But there in the exray on his back was a slug still in his body. His entire life he was afraid of loud noises... we assume his old owner took him out to the desert to kill him, but he got away.

IMHO.. i would always adopt a dog before buying one.