Sentence:  The intrepid DIY builder may be the wrong person to convince, as several posters suggested. / "I'd compliment his ambition, all the time pushing for what the family wants/needs (more house now)," says PeterJ. "I'd be looking Mom in the eye when I paint the picture of Dad with no time. . . . Plant the seed, let her do the 'convincing.'" / JoeH has the same idea. "Build what he wants," he says. "After wife notices you did the shell in a (pick a day), she'll be your <WSD>inside salesman</WSD>." 
Meaning of the word marked with <WSD> tags:  Someone employed within the organization of a prospective buyer who serves (whether for good motives or not) as an advocate for a seller's product or service.