Nick Cannon & Mariah Carey Selling Home -- Anybody Want A $13M Mansion? (PHOTOS)

Mariah Carey and Nick Cannon aren't packing up their toothbrushes just yet ... their Bel Air mansion -- which they thought was sold -- is BACK on the market, TMZ has learned.

Real estate sources tell us an out-of-state-buyer had negotiated a deal to buy the pad for about $10M -- it had been listed for just under $13 mil -- but had a change of heart.

We're told the client didn't give a reason -- just simply backed out of the deal.

As we told you ... the house sits on 3 acres and has a movie theater, a gym, an indoor basketball court and a putting green.