Several sellers have put up the limited-edition Istana model set made of Lego pieces for sale on online marketplaces Carousell and eBay for between $100 and $300, the day after its launch.
The model of the main building was unveiled on Wednesday at the Labour Day Istana open house by President Halimah Yacob, to commemorate the Istana's 150th anniversary this year.
Visitors who made a minimum donation of $70 to the President's Challenge could redeem a set.
All 500 sets were fully redeemed within two hours. The model was commissioned by the President's Office and designed by a Lego certified professional.
A check by The Straits Times showed at least 13 sets up for sale on Carousell on Thursday evening, with prices ranging from $130 to $210. There was at least one listing on eBay priced at US$199.99 (S$272).
Two sets that were priced at $138 and $150 were later marked as sold.
One Carousell seller said he is not a Lego fan, but had wanted to catch a glimpse of Madam Halimah.
The 21-year-old student, who wanted to be known only as David, said: "I got one out of curiosity. When I realised how limited it was, I decided to sell it on Carousell."
He had given a $100 donation for the set.
He sold it for $150 on Thursday.
Toy brick models of the Istana are proving popular.
Artist Jeffrey Kong, 39, who is behind the brand Artisan Bricks, posted on Facebook pictures of his version, and said in his post that he was "pleasantly surprised by the support and enquiries I got".
He told The Straits Times that there had been some hype about the Istana model prior to the launch, but "it is the same with any new exclusive models", and occurs among the same pool of adult collectors.
He compared it with the Ministry of Education giving out free commemorative SG50 Lego sets to students in 2015. "If you find people selling the SG50 sets, nobody is buying because it (the hype) has piped down," said Mr Kong.