1) NOC from builder is required for purchase of flat from regd owner
2) since property is under construction you would need tripartite agreement to executed between seller , purchaser and builder.
3) builder can refuse to record your name as new owner in his records
4) if you purchase property without builder consent your name would not be recorded as owner by the builder . it is in your interest to wait till construction is completed and Oc issued and society formed of flat owners
5) society cannot charge more than Rs 25000 as transfer charges
6)resale of a property within one year of buying it from a developer would result in the state collecting stamp duty only on the profit from the transaction.