I’ve seen some libertarians who give the Law Merchant as an example of an autonomous system of law that evolved outside the state.

One libertarian thinker, “quasibill” of The Bell Tower blog, takes a very minimalist view of the amount of contract law that could be coercively enforced. In the case of bankruptcy, for example, he starts from Rothbard’s assumption that compelling specific performance is a form of slavery. But he takes it further, arguing that it is virtually impossible to prove, in the event of default, fraudulent intent at the outset. So essentially, in almost all cases, he would regard arranging adequate security, requiring default insurance on the debtor’s part, etc., as the creditor’s responsibility, and consider default just another part of the business risk.