New York Post Publisher Rupert Murdoch yesterday announced he will purchase the financially ailing Buffalo Courier-Express if its nine unions agree by midnight Thursday to a one-third reduction of the paper's work force.

Under the terms of the conditional sale, Murdoch's News America Publishing Inc. will pay nothing for the 126,000-circulation morning paper, but will assume most of its liabilities.

The paper has been losing an average of $8.6 million a year since 1979, when Cowles Media Co. of Minneapolis reportedly paid $30 million for it and a local cable television system. The TV property is not involved in the proposed sale.

Last week, executives of Cowles Media, which has newspapers and broadcast and cable properties in eight states, had announced that the paper would publish its final edition next Sunday unless a buyer could be found.

The announcement prompted a reopening of talks with Murdoch, who had earlier expressed an interest in buying the paper but backed off when he could not get the union concessions he is seeking.

Over the weekend, a delegation of Buffalo political and business leaders, including Mayor James D. Griffin and Courier-Express President and Publisher Roger P. Parkinson, met with Murdoch in New York to ask him to save Buffalo from becoming a one-newspaper town.

In a statement issued yesterday, Cowles Chairman Otto Silha hailed Murdoch as a publisher of "great competence" who would would provide a "lively" newspaper. In addition to the Post, Murdoch's publications include the Village Voice, New York Magazine, The Star celebrity tabloid, as well as papers in Texas, Australia and England.

Negotiations between Murdoch and the nine Courier-Express unions representing 1,100 employes began last night. If they are successful, the paper will follow a pattern similar to the New York Daily News. That paper was on the sales block and appeared near death earlier this year, but it stayed in business when its owner, the Chicago Tribune Co., won substantial givebacks in staffing levels from the unions.

Such eleventh-hour rescues have been the exception rather than the rule. Over the past 20 months, according to the American Newspaper Publishers Association, 14 newspaper around the country have folded and another 11 have merged.

Indeed, just yesterday there was action on two other fronts involving newspaper mergers:

* In Seattle, the Post-Intelligencer carried a front-page message asking its readers, advertisers and employes to support the paper while it pursues a proposal to operate jointly with The Seattle Times.

The plea followed an announcment last week by the 9th U.S. Circuit Court of Appeals in San Francisco that it would hear the paper's appeal of a lower court ruling this summer that blocked its proposed joint operating agreement.

The Newspaper Preservation Act of 1970 exempts newspapers from some antitrust laws in order to preserve the editorial voice of a failing newspaper. Under the proposed agreement, which had been approved by Attorney General William French Smith, the profitable Times would take over all of the operations of the Post-Intelligencer except the news and editorial departments.

* In Oakland, the publisher of the Tribune announced that starting Oct. 25, it will switch to morning publication in combination with its sister paper, Eastbay Today.

Both papers are owned by the Gannett Co., which began publishing the morning paper when it purchased the Tribune in 1979.

Tribune editor and publisher Robert Maynard described the merger as an advance rather than a retreat. He said the step was taken because the paper for the first time has adequate press and mailing facilities to handle a full circulation effort in one cyle. At present, the two papers are put out by the same staffs and often carry identical articles.