Sales of existing homes from July through September slipped 2.5 percent from their level a year earlier and will remain slow in the first half of 1991 because of the weakening economy, a real estate trade group reports.

The National Association of Realtors said sales of previously owned homes totaled a seasonally adjusted annual rate of 3.66 million units in the three-month period. Sales declined in the year's two previous quarters as well.The Realtors recorded sales of 3.78 million last year, down from 3.95 million in 1988.

Although there were areas of strength in the housing industry during the third quarter, sales declined in 28 states and were unchanged in Oregon. Nineteen states posted increases. Statistics were unavailable for Alaska and Maine.

Overall, however, "the market will remain slow for the rest of 1990 and into the first half of 1991, as the country experiences a mild recession," Realtors President Norman A. Flynn said.

The Realtors also reported the median price of an existing home was $96,600, 1.8 percent above that for the third quarter of 1989. It ranged from $375,000 in Honolulu to $49,200 in Saginaw, Mich., among the 96 metropolitan statistical areas surveyed.

The median price means half of the homes cost more, half less. The survey includes detached homes, town houses, apartment condominiums and co-operatives.

The only region experiencing increased sales was the South, where sales rose 1.4 percent to an annual rate of 1.42 million. Several oil patch areas that had suffered adverse market conditions in recent years recorded increases.

Sales in the West fell 8.8 percent to an annual rate of 580,000 and in the Northeast were down 8.5 percent to a 650,000 rate.

Tuccillo said the decline in the West resulted from a sharp drop in coastal California markets. The median price dropped 5.3 percent to $138,200, reflecting more purchases in less costly, inland areas.