noun which may take the genitive case particle 'no'
interpersonal; between individuals
Used as a no-adjective to describe something happening or existing between private individuals, as opposed to between organizations or in public contexts.
個人間の貸し借りはトラブルになりやすい。
Lending and borrowing between individuals can easily lead to trouble.
個人間売買のサイトを利用する。
I use a site for person-to-person buying and selling.
対人 is also 'interpersonal' but often used in contexts like 対人関係 (interpersonal relationships) or 対人恐怖症 (social anxiety). 個人間 emphasizes the private, non-institutional nature of the interaction.
Compound of 個人 (individual) and 間 (between, among).