They do get people calling them to find them vacations. Thank them for the offer and just turn them down. If you have a place reserved for rent, and they have someone looking for that time frame, than hook the vacationer up with a place for more money.
Supply, season, and demand is the key, If I were looking and I called a travel agent and they found a unit but cost more because I didn't find out till later I could go I know it is going to cost more money. I'd be happy they found one.
Dave