One needs to understand factual scenario, refer to the 'agreement to sell' (i.e. between 'first buyer' & 'builder') as well as so-called re-sale agreement (i.e. between 'first buyer' and 'second buyer') to give a comprehensive answer to the query raised.
However, followings points - generally specking - can be looked into:
A. Sale of under-construction property is NOT covered in Para No.5 (b) of Schedule-II from point of 'first buyer'.
B. 'Builder' continues to be liable to pay GST on his 'construction services' irrespective of so-called re-sale agreement (i.e. between 'first buyer' and 'second buyer').
C. 'Rights of First Buyer' (i.e. under-constructed property till date of re-sale agreement & further construction till completion, generally specking) is transferred to 'Second Buyer'.
C1. Tax Dept. may try to levy GST alleging that this is nothing but provision of service.
C2. As 'First Buyer' is not providing 'construction services' to second buyer and his rights in 'under-constructed property till date of re-sale agreement' is getting transferred to second buyer, benefit from Para No.5 of Schedule-III should be claimed by the 'first buyer'.
C3. This may become an area of dispute including on point of valuation as well as 'classification' of 'supply' by first buyer to second buyer.
D. Alternatively & assuming that this is only one-off & unplanned transaction for the first buyer, subject supply from first buyer to second buyer is NOT in the course or furtherance of 'business' from point of 'first buyer' but something which was not planned initially but forced into due to changed personal circumstances. And hence, no gst can be levied on the 'first buyer' as there is no supply u/s 7(1)(a) of the CGST Act, 2017 by the 'first buyer' .
These are ex facie views of mine and the same should not be construed as professional advice/suggestion.