Danger Will Robinson...although that bike looks like it has a Thudbuster suspension seatpost, that doesn't qualify it as a mountain bike. The seller is either lying about it's capabilities or doesn't have a clue.

The missing rear wheel is also fishy. It's possible the seller is a thief who walked away with somebody's new bike & front wheel because the bike had been locked only by only the rear rim...of a wheel that had quick-release axles.

Just sayin'...approach this one with caution.