Is it Mandatory to get NO Dues & NOC from Purchaser

HI Experts, While purchasing a Flat in Delhi, as per seller, socity management is asking for some amount whithout which they are denying to issue NOC or NO dues certificate. Now those certificate are not required for Bank loans and I do understand thats important for my own reference. However in my exp (with purchase of another property) even with No Dues certificate, there are loopholes which socity tend to exploit for any past incurred expenses stating when the NOC/No-dues were issued accounts were not audited etc. or BSES/Jal board had not send any bills. Based on my exp socity management again will ask some "transfer fee" / "entry fee" while doing membership transfer which then again I have to pay (unofficially) Anyhow resummerizing following are my queries A) Does those NO Dues/ NOC has any ligal significance in case situation comes where we have conflict regarding past outstanding payment for the cases mentioned above ? B) Do the seller really Need NOC for sale/purchase of flat ? C) If I donot get NOC from Seller do the socity have any rights to restrict my entry to the socity after registry ? D) Is the No dues certificate presented to DDA during the freehold conversion have any significance for me in case socity present any outstanding amount. E) Do the socity have the rights to deny transfer of membership and is it legal for them to ask for transfer fee ?