This is my response to you:
1. The property can be sold between the guidance value and the market value;
2. The property cannot be sold less than the guidance value;
3. The builder is doing so to save on tax implications;
4. You on the other hand will have to pay stamp duty at guidance value;
5. You can also approach a CA and ask about the tax implications of the same;
6. Not only seller but also the buyer will be at loss if he/she agrees to buy (makes agreement of) at a value below the Stamp Duty Value because of the operation of Section 56(2)(vii)(b) of the Income Tax Act, 1961. According to the said section if the stamp duty value exceeds the purchase consideration by more than Rs.50,000 then, the difference between the Stamp Duty Value and the Purchase Consideration will be treated as Income u/h Other Sources.