SL Exchange is a huge marketplace for SL inworld items. It combines shopping on a website with automated inworld delivery of goods and services. SL Exchange run a large number of self-service terminals and warehouses where sellers can deposit their items. They also offer a Linden $ exchange and payout.
SLExchange was, at a certain time, partly acquired by Linden Labs and subsequently renamed XStreetSL. In a further step, XStreet was closed and all traffic is being redirected to the new exchange for virtual items called marketplace.secondlife.com