Virginia's economy slipped once more in September, according to the latest edition of the Virginia Business Report from the College of William and Mary in Williamsburg.

"A broad measure of performance is the indicator of bank debits," says Leland E. Traywick, director of the college's Bureau of Business Research and editor of the monthly report. "On a dollar-volume basis, almost 85 percent of all business transactions are done by check."

In September, checking account transactions were up 15.7 percent over those of September 1981. "When August 1982 was compared with August of last year, bank debits rose 21 percent," Traywick adds. "Thus, some slowness has developed this fall."

Consumers just are not spending very many dollars. Retailers had hoped that back-to-school sales in September would bring the shoppers back into the stores. Despite an increase of 4.3 percent in newspaper advertising lineage, September's retail sales rose only 2 percent compared with September of last year. With the current inflation rate, "real sales" actually reflect a minus 4 percent change from last year.

The bright spot in the Virginia economy was the change in dollar value of building permits. Permits rose 40.3 percent in September compared with the same month last year. This is the greatest increase so far this year, but building permits as an economic indicator have been volatile, dropping 39 percent in May, rising 7.1 percent in June, rising 34 percent in July, and dropping 2.4 percent in August.

Electricity consumed in Virgina increased slightly. It was up 2.8 percent over last September. On a year-to-date basis, it is running at an increase of 1.5 percent. September was a little higher than usual. Summer and air conditioning bring the greatest demand, but the festivities of December put that month not far behind.

New car registrations were down again, by 13.6 percent. They have risen only during one month this year, in June, and then only by 0.1 percent. Presumably, inventories of 1982 and 1983 models are overlapping, and business is suffering.

Water consumption was down 0.3 percent and postal receipts were up by 14.6 percent in September.

The nonagricultural employment data for September has not yet been released by the Virginia Department of Labor and Industry. On the basis of January through August, the increase has been 0.2 percent. Some months have even been slightly negative.