You can sell your old office furniture on places like Craigslist, eBay, Facebook Marketplace, and other resources. There are also various third-party companies that will buy furniture from you to sell it to other companies in need, usually at cheaper price.
If you don’t want to sell your old furniture or don’t think that it will yield a lot of money and not worth the effort involved, you can always donate it to a charity of your choice. Local charities and nearby thrift stores usually take all sorts of furniture. Sometimes charities and thrift stores will come to your office and pick up the furniture as well.