Other sales is assumed to mostly consist of advertising (used by sellers and brands). It also includes other things like cobranded credit card agreements, but those are expected to be a minor part.
Amazon defines this as:
Includes sales not otherwise included above, such as certain advertising services and our co-branded credit card agreements.
2014: $1.322 billion, 2015: $1.710 billion, 2016: $2.950 billion.
Last reported quarter 2019 Q1 it was $2,716.00 million, up by 34% year-over-year from $2,031.00 million. From $4,653.00 million in 2017 it increased by 117% to $10,108.00 million in 2018.
Source: Amazon Quarterly Results.