In Belgium we have 2dehands. Not perfect or open source but miles better than Facebook marketplace.
No payments go through the platform. People meet in person and use cash or use a bank transfer or 3rd party payment like payconiq to buy and sell things. It works perfectly.
Not everything has to have an integrated shopify style payment platform. Most of the population is perfectly capable of exchanging money.
If you aren’t the customer, you are for sure the product. Even then, sometimes you are both.