Here is a neat example of a coated postal card used by a commercial concern soliciting for spoiled and unused postal cards.
This particular merchant used a coated card to advertise that he was both in the market to buy spoiled cards, as well as offering to coat them for prospective customers. The card itself served as a sample of his product in the latter case.
This card is an interesting example from Chicago of an early-issue card being 'recycled' in this fashion, by bronzing. The card was formerly soiled and preprinted on reverse by 'Middleton & Co.', a merchant out of Savannah, GA.
Place mouse on image to see the reverse side of the card.