It's not really an auction ... more like a raffle. We got Billy Baldwin at LAX Tuesday afternoon and he straight-up told our photog he wants to raffle off the plus-1 invitation to his niece and the Biebs' wedding bash, and he promises it could be for a great cause.

As we reported ... Justin and Hailey have already tied the knot -- sans prenup -- but they plan to have a formal wedding ceremony down the road. Billy says he could make a fortune off the ticket, and all the money is going to, in his words, a kickass charity.