Diamond Engagement Rings?

I don't think that's likely to happen if you use a reputable jeweler who has been in business a long time. Ask your married friends where they got their rings and shop at those stores. It's best to go on a recommendation. I wouldn't buy from some guy with no office, that's shady and like you said, you have no way of verifying that the certificate he gives you is for the diamond you're buying, because you're not an expert.