Whether we need to get sale deed done

Contact a local advocate and file a declaration suit in a competent court involving all flat owners.

You need to havev a properly stamped, executed and registered sale document to prove the absolute ownership of the flat. 

In the absence of sale deed you have "no" title (ownership) of the flat despite the fact society conveyance deed has record of the ownership (title).

There is no such concept as "deemed conveyance" in Transfer of Property Act, 1882 as mentioned by you.

You are not the owner of flat then what can be sold by you ?

Ask the builder to execute proper "Sale Deed" in favour of purchasers, if builder is reluctant / non available file a suit for specific performance against the builder, society and Sub-Registrar or suit for declaration through a local prudent lawyer.