As with gold, there is also a difference between the purchase and sell price of silver (bid and ask price). This difference is known as the spread and arises from the margin applied by suppliers and dealers, as well as the market dynamics of supply and demand.
This is why you receive slightly less than the current rate when selling, and pay slightly more when buying.
Have you, besides silver, also bought gold and/or platinum via GoldRepublic? The buyback guarantee also applies to selling gold and selling platinum, provided you purchased these precious metals via GoldRepublic.