ないものでもない

ないものでもない is a guarded yes: "it's not out of the question", "I wouldn't say I won't". It follows the ない-form of a verb, often a potential one, and a condition usually comes first with ば, なら or しだい次第で.

Reluctant willingness

With a verb of the speaker's own choosing rather than a potential, it says the speaker might agree, but is not offering. It leaves room for negotiation.

Register

ないものでもない is stiff and mostly written. In conversation, ないこともない or なくもない says the same thing, and the spoken contraction ないもんでもない is also heard.