Property agreement - stamp duty not paid and not registered

M/S XYZ Pvt ltd ( the last purchaser) wants to sell Residential property . The property is located in Mumbai.

Background

1.M/s ABC Pvt Ltd has developed the property.

2.M/s ABC Pvt Ltd allotted 35% area to Mr. B.

3.Mr. B has sold flat to Mr.C

4.Mr. C sold to flat to M/s XYZ Pvt ltd .  M/s XYZ Pvt ltd is last purchaser and it intends to sell the property.

However they have following query.

Here agreement mentioned at point number 2 (i.e. between .M/s ABC Pvt ltd and Mr.B ) and point number 3 ( i.e. Mr.B and Mr.C ) ---These 2 agreements are not registered at sub registrar office and stamp duty not paid at all and hence these 2 agreements are not available. Only last Agreement is available i.e. between Mr. C to M/s XYZ Pvt ltd.

Kindly guide what is the consequences for future purchaser .

Thank you in advance.