Real or Fake Macbook Pro

Are there fake Macbooks with original serial numbers being sold in the market?

Possibly. It may have been stolen, or (more likely) the offer may be an attempt to defraud you. Nobody sells legitimate Apple products at prices substantially less than Apple's online prices. Discounts of 5% are common, but 50%... never.

The serial number given at the back of the box is legitimate ...

Authentic Apple boxes are commonly available on auction sites. If you wonder why people sell empty boxes, now you know.

In the United States we have a saying: if it seems too good to be true, it probably isn't.

In Latin, caveat emptor.