Answer:
no double coincidence of wants is required. People with different interest can work together. As Claire want's to work but do not want their entire salary in tomatoes.
Explanation:
Under a bartering system Wendy should look for a person willing to work for tomatoes or other of her belongings else, they will not accep the job.
As there is a currency economy Wendy pays Claire with it and Claire can then spend on tomatoes or, any other good she consider to maximize her utility.
While Wendy can sale the tomatoes anywhere else thus, amking a gain right away. If Wendy was required to look only for tomatoe interest some portion of their harvest could be lose while looking ofr commercial parties.