He aint been doing shit for my fantasy team and now I know why…

Indianapolis Colts running back Trent Richardson has something in common with Kim Kardashian — ’cause someone’s trying to cash in on a sex tape featuring the NFL star … TMZ Sports has learned.

We know … someone is shopping explicit footage showing the former 1st round NFL Draft pick engaged in an orgy with at least 3 women.

The person who has the footage has already contacted various media outlets, including TMZ Sports … but so far, no takers.  We’ve seen the footage — let’s just say Trent’s got a lot in common with the Colts mascot, if you catch our drift.

We contacted Trent’s people for comment multiple times — so far, no word back.