is sale agreement between builder and seller registered? if possession letter issued that means Oc is granted by muncipial corporation . construction has been completed . seller has to obtain approval from builder for sale of flat . please note that before you finalise deal please cross check with seller who will pay the transfer charges . builder will charge over a lakh to transfer flat in favour of purchaser . better wait for society to be formed .
transfer charges are lower in society . anyhow agreement will be between you and the seller .