Who could use surplus treaty-era warships

Would surplus warships have been too old to matter if the Washington treaty allowed their sale?

No; although their remaining service lives would be limited, older dreadnoughts and cruisers could still be useful to the powers buying them.

Older battleships and cruisers could still be useful to powers seeking to secure their position after the First World War. Turkey, Greece, and China are suggested as possible buyers of surplus ships, including battleships armed with 13.5-inch guns. Age would shorten their remaining service, but they would still provide useful capabilities for those purchasers. Brazil's acquisition of other countries' retired amphibious ships offers a modern comparison: ships released by one navy can help another build up a substantial force.