Final answer:
The ad can indicate that the seller holds a real estate license by including the phrase 'seller holds a real estate license' in the ad.
Step-by-step explanation:
The ad can indicate that the seller holds a real estate license by including the phrase 'seller holds a real estate license' in the ad. This is a straightforward and clear way to inform potential buyers that the seller is licensed to sell real estate. It is not necessary to attach a copy of the seller's real estate license or include a link to the license in the ad, as simply mentioning that the seller is licensed to sell real estate is sufficient.