If i want to sell my diamonds that are not certified, is it of lower quality or lower value than a certified diamond?

Yes and no. Any diamond can be certified, even those diamonds of poor quality. Having a low-quality diamond certified does not increase its quality at all. Likewise, certifying a high-quality diamond does not make it any better. However, because the certification process costs money and certified diamonds are more desirable on the open jewelry market, a certified diamond is more valuable than a diamond of the exact same quality that does not have a certificate.