NFL Star Brandon Marshall -- I'm Selling My Illegal Green Shoes

Chicago Bears wide receiver Brandon Marshall is taking it one step further with his infamous green shoes -- after disobeying the NFL's direct orders NOT to wear them on the field last night -- TMZ has learned, he's auctioning them off ... for charity.

Brandon announced he'd be wearing the green shoes during last night's game against the Giants -- in honor of Mental Illness Awareness Week -- but the NFL said he'd be fined if he did ... because the shoes violate the League's uniform policy. (The only exception is the pink shoes players are currently wearing to support breast cancer awareness.)

As you can see, Brandon (who was diagnosed with borderline personality disorder in 2011) followed through with his promise ... and said he'd match the NFL's fine with a donation to a breast cancer charity. But he's not stopping there.

We've learned Brandon has now decided to auction off the green cleats, and donate the proceeds to his own foundation, dedicated to helping people cope with mental illnesses like his own. The auction is currently at $2,000.

FYI, if you're wondering how much Marshall could get fined by the NFL -- it could easily exceed $10,000.

Marshall was already fined $10,500 last year for wearing orange cleats -- not for any particular cause -- but since last night's stunt would be a repeat offense, the fine could be larger. It's expected to be handed down next week.