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 2017 Q3 it was $1,123.00 million, up by 58% from $709.00 million the same quarter a year before.
Since 2015 it increased by 74%, from $1,697.00 million to $2,951.00 million in 2016.
Source: Amazon Quarterly Results.