According to Official Guinness Records,
The most watches sold on a single online platform in 24 hours is 1,112,561 watches and was achieved by Tmall (China) during their Single's Day sales on 11 November 2015. This was one of 9 records set during Single's Day on Alibaba's online retail platforms.
For a complete list of 2015 records, please visit 2015 Guinness Records in China.