Japanese oil companies have resumed negotiations for purchases of more than half a million barrels a day of Iran's high-priced crude, Iranian government oil sources said today.

The talks resumed despite Japan's agreement in principle to apply the same sanctions adopted yesterday by the nine nations of the European Common Market.

Last month Iranian Oil Minister Ali Akbar Moinfar said Iran would refuse to sell oil to any nation applying sanctions against it.

Oil purchases were officially exempted from the European sanctions. But a gentleman's agreement amounting to informal sanctions has discouraged buying Iranian crude for the past month. The main objection has been that Iran's $35-a-barrel price was far higher than that charged by other members of the Organization of Petroleum Exporting Countries for comparable oil.

Considerable optimism was expressed in National Iranian Oil Co. circles that the dozen Japanese companies now negotiating would be compelled to accept the full Iranian asking price. The price led to suspension of sales negotiations with the Japanese firms in mid-April.

Resuming sales of the 530,000 barrels a day once sold to Japan would almost double Iran's crude exports and provide badly needed foreign exchange to keep the sagging economy going.

Buoying Iranian oilmen were the latest price increases by Saudi Arabia and Mexico, sources said.

Although Iran says it exports nearly 1 million barrels a day, oil analysts can trace only 611,500 barrels a day in term contracts, and Iranian officials say no more than 100,000 barrels a day is sold on the spot market.

Japanese companies, British Petroleum and Royal Dutch Shell together purchased more than 800,000 barrels a day until March 31.

Now about a third of Iran's oil exports are believed to go to communist countries.

China, the most recent customer, signed up for a 30,000 barrel a day deal.

Developing nations account for about half Iran's sales and the rest goes to neutral Western European countries such as Finland and Sweden.

Meanwhile, Iran has begun canceling agreements with its traditional Japanese and Common Market member banks for oil revenue handling agreements. j

Their place is being taken by two Swiss banks, one Austrian and one Swedish bank and the offshore branch of the State Bank of India located in Bahrain.

The move was dictated by fears that economic sanctions could eventually include further controls by U.S. allies on Iran's foreign-held assets.