three party transaction for buying a flat

Can someone please highlight the differences between buying a new apartment directly from a builder v/s buying from an investor who has not registered the UDS. What are the things to look out for when doing such a transaction? 

if i get a resale offer for a new under construction apartment that is almost ready to occupy, & the seller has a booking agreement with the builder and he has been paying them as per schedule, however he has not registered the UDS in his name...Now he wants to sell the apartment and book profit. if he says the 3 parties - buyer (me), seller (him) and builder can enter into a tri-party agreement which will mention that he is doing a sale of the apt booked by him and request the builder to register the UDS directly in the buyer's name (i.e. my name), and he is in good terms with builder and therefore they will co-operate without any issues...He also mentions that the bank should disburse the loan amount in his name and not to the builder this legal??