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A free online marketplace to buy and sell goods / services using Bitcoin. OpenBazaar is a peer-to-peer ecommerce platform with no fees or restrictions.
BTCPay Server is a self-hosted, open-source cryptocurrency payment processor. It's secure, private, censorship-resistant and free.
If you regularly sell things you can accept payment in BTC without relying on a 3rd party using https://btcpayserver.org/.