The housing market has become so expensive that some sellers are offering to help pay their buyers’ expenses, including their mortgage.

With mortgage rates at a 23-year high and home prices staying put, housing affordability has worsened to the lowest point since 1985, when rates were in the double digits. Sales of previously-owned homes are on track to end the year at a pace that’s the slowest since the Great Recession in 2008.  

The…