If the seller is liable but fails to pay seller's stamp duty (SSD), the Agreement between you and the seller is not considered as duly stamped even though you have paid the buyer’s stamp duty. Therefore, you should ensure that the seller pays the SSD.
SSD is payable if the seller has bought :
(i) Residential property between 20 February 2010 and 29 Aug 2010 and sold it within 1 year from the date of purchase; or
(ii) Residential property between 30 August 2010 and 13 Jan 2011 and sold it within 3 years from the date of purchase; or
(iii) Residential property between 14 Jan 2011 and 11 Jan 2012 and sold it within 4 years from the date of purchase;
(iv) Industrial property on or after 12 Jan 2013 and sold it within 3 years from the date of purchase.
Before the sale completion, you may check with the seller's lawyer who may make a search on the property to ascertain the date of purchase by the seller. Whether the seller is liable for SSD and the amount of SSD payable would depend on the date of purchase and the date of sale.
You may ask for a copy of the stamp certificate from the seller's lawyer as proof that SSD has been paid. If the seller has not paid the SSD by the sale completion date, you may consider withholding the stamp duty amount payable by the seller plus any penalty chargeable.