His gaze was charged with a secret meaning.

He seemed to be suggesting unspeakable matters to Priam.
That bright face wore an expression which such faces wear on such occasions--an expression cheerfully insinuating that after all there is no right and no wrong--or at least that many things which the ordinary slave of convention would consider to be wrong are really right.

So Priam read the expression.
"The dirty rascal wants me to manufacture imitations of myself for him!" Priam thought, full of sudden, hidden anger.

"He's known all along that there's no difference between what I sold him and the picture he's already had.

He wants to suggest that we should come to terms.