The University of Virginia has set up an institute to provide a third-party mediation of environmental disputes involving air and water quality, neighborhood zoning and site location. The new Institute for Environmental Negotiation is headed by Richard C. Collins, chairman of the university architectural schools urban and environmental planning department. The institute's first assignment will be to mediate a roundtable on toxics called for by the Du Pont Co. and the Environmental Defense Fund.

District residential sales in January averaged $108,792, compared with $84,642 in January 1980, the Rufus S. Lusk & Son publishing firm reported. But the total number of sales in the city decreased 18 percent, from 400 to 327, and the total dollar volume decreased from $37.8 million to $35.6 million, Lusk reported.

Better Homes Realty Inc. has acquired the property management department of Joseph W. Seay Co. Broker Joseph W. Seay and his son, Joseph Jr., now are associated with Better Homes at its corporate headquarters in Arlington. Joseph Seay was 1980 Virginia Realtor of the Year.

A booklet on how to save money when you hire a real estate broker, written by local broker Peter G. Miller, has been published by the Springfield Press. Copies are available for $3.50 from P.O. Box 1762, Silver Spring 20902.

The Washington Building Congress will sponsor a seminar Wednesday on how to start a project profitably, starting at 8:45 a.m. at the new Washington Marriott at 22nd and M streets NW. James J. Tansey will moderate the program. Information or the session is available by calling 483-9484.

The Fred Segesman Inc. realty firm in Montgomery County has joined the Better Homes and Garden real estate network by affiliating with Holley, Hargett & Spain Realtors, a Northern Virginia firm. The Holley, Hargett firm represents Better Homes and Gardens in the Washington area and now has 12 affiliated offices.

Montgomery College will offer a short course on condominium and cooperative buying and selling. The non-credit course will meet Wednesday evenings, beginning next week, at the college's new center at 7815 Woodmont Ave., Bethesda. Tuition is $20. For information on this and other real estate courses, call 656-7482.

A series of classes and workshops on solar energy will be offered within the next several weeks at several locations by Solar Science Industries Inc. Information is available by calling 474-6511.

The Property Management Association of Metropolitan Washington will sponsor a day-long seminar on practical approaches to commercial space planning, layout, design and leasing. The session will be held May 12 at the Bethesda Marriott. For information or reservations, call 587-6543.

Attorney and Washington Post real estate columnist Benny L. Kass will conduct a workshop on creative financing techniques from 9 a.m. to noon March 21 at the Wellington Hotel. The cost is $40 per person. For information, call 966-5703.