Any affirmation of fact or any promise by the seller relating to the thing is an express warranty if the natural tendency of such affirmation or promise is to induce the buyer to purchase the same, and if the buyer purchases the thing relying thereon. No affirmation of the value of the thing, nor any statement purporting to be a statement of the seller's opinion only, shall be construed as a warranty, unless the seller made such affirmation or statement as an expert and it was relied upon by the buyer. (n)