Davis Advertising has sold its office building on Grafton Street in Worcester for just over $2 million and will lease back about half of the building. Davis, the largest advertising firm in Central Massachusetts with 28 employees and $19 million in annual billings, sold the building to the family operating the professional staffing services firm Masis Staffing Solutions. The deal was announced Thursday by the Worcester realty firm Kelleher & Sadowsky, which represented both buyer and seller. The deal closed Wednesday. Masis Staffing Solutions, which is now based at 416 Belmont St. in Worcester, will move its headquarters to the remaining space in the building it'll share with Davis.