Sales of existing homes surged 12.1% last year.

The National Assn. of Realtors reported that 3.22 million single-family homes were sold in 1985, the first 3-million-plus sales year since 3.83 million homes were sold in 1979. Analysts credited the increase in sales to falling mortgage rates and a big pool of homeowners who traded up to larger homes. The survey also disclosed that the median sales price for an existing home climbed to $74,800 in December, up 3.7% from December, 1984. The annual increase in sales followed a 5.5% rise in 1984, when 2.87 million homes were sold. For December, existing home sales rose 3.2% above the November level.