According to Official Guinness Records,
The most cars sold on a single online platform in 24 hours is 6,506 cars and was achieved by Ali Car (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.