Avoid sellers who do not offer returns if an autograph cannot be verified authentic by legitimate service. You will avoid problems down the road. Especially if that is important to you.
Joe W is correct. There are certain sellers who claim to offer a guarantee on authenticity, but then they do not accept the opinions of reputable independent authenticators and they claim they know more than anyone else. In short, to get a refund, they would have to disagree with themselves. Read the fine print and avoid sellers who offer false authenticity guarantees.
What Steve says is true. I'm not getting into names but I've heard "Yes, well, I can't get MY money back so..." from sources you'd not expect. Yes, I did prevai, by finding a new diagnostic.