Do I pay capital gains tax when I sell the house I have lived in for 40 years?

Four decades of ownership usually means significant appreciation, so capital gains tax becomes a real question once your profit, the sale price minus your original purchase price and improvements, is compared against the federal primary residence exclusion available to qualifying owners. That comparison is worth running before you set a list price, not after. Confirm your specific numbers with a CPA so pricing decisions account for any tax due.