Self vs. other enforcement is less important than consequences, which are the same in both situations. If a country has the opportunity to negotiate the terms of trade, that is clearly preferable to terms being imposed unilaterally by others.

Yes, countries can break agreements, but good agreements provide benefits to all parties, such that breaking an agreement results in reciprocal loss of benefits. There are always edge cases, but by definition, not everything is an edge case and there are countless routine, negotiated agreements which benefit everyone and are happily followed.