Answered my own question.
After getting the buffer geometry, run a spatial selection using a querytask :
mPolygon is the geometry of the buffer I already did.
//set up query task against layer, specify the delegate
self.queryTask = [AGSQueryTask queryTaskWithURL:[NSURL URLWithString:countiesLayerURL]];
self.queryTask.delegate = self;
self.query = [AGSQuery query];
self.query.outFields = [NSArray arrayWithObjects:@"*", nil];
self.query.returnGeometry = YES;
self.query.geometry = mPolygon;
self.query.spatialRelationship = AGSSpatialRelationshipIntersects;
[self.queryTask executeWithQuery:self.query];