At first glance, a help-wanted ad posted on online job boards last week for a New York-based media buyer looked like thousands of others placed by Madison Avenue each year: "Requirements. 3+ years experience in media buying for a large advertiser or agency. Strong negotiation and analytical skills. Proven track record of exceeding goals." What made it remarkable was its sponsor--Google--and its timing, coming just months into a "test" the Internet search engine has been conducting, in which it buys and resells advertising space in offline media. To date, Google executives say it has been just that: a test. They also say it has been confined to print media, though they are evaluating whether to expand into other media as an extension of what they've been doing online all along: using Google's technology to serve relevant, targeted ad messages on behalf of advertisers and agencies. But that doesn't explain the growing sense of angst among some of Madison Avenue's top executives when they allude mysteriously to Google's plans for muscling into the media planning and buying business, fearful that the highly capitalized company plans to muscle into Madison Avenue's turf.