There are plenty of upsides to marketing jewelry and watches bearing a retailer’s own brand nameBig name brands like Tiffany & Co. and Cartier might get customers’ attention, but retailers in the know say there’s a cachet that comes from having your own in-house brand of jewelry or watches. Private-label lines offer retailers a brand-building boost along with the ability to tailor products to their customers’ specific tastes. First and foremost, private labeling gives retailers “an exclusivity by having their own private brand for the store,” says Alan ­Grunwald, president of Belair Time Corp., in Lakewood, N.J., a supplier of ­private-label watches to jewelry retailers. “It has a lot of meaning if they have a good reputation.”A private-label collection is a unique opportunity to convey your store’s personality to the buying public in a more tangible way than