Editor ed = Application.DocumentManager.MdiActiveDocument.Editor;
Matrix3d matUCStoWCS = ed.CurrentUserCoordinateSystem;
Point3d startPointWCS = startPoint.TransformBy(matUCStoWCS);
Point3d endPointWCS = endPoint.TransformBy(matUCStoWCS);
Vector3d acVec3d = startPointWCS.GetVectorTo(endPointWCS);
poly.TransformBy(Matrix3d.Displacement(acVec3d));