Many of these services, like Alibaba, NewEgg, (even GameStop), and old-time catalog services, *Dr. Nimrods health supplies and the random Elvis painting or dragon statues type trash that Tom Bosley used to pitch for (on US TV) are what in business terms are called drop shipping sales. These were popular, as they were a way to offer many items without putting out the overhead for the stock. This has become almost a common business practice today.
In years gone by catalogs were put together with your name (as a retailer) on them, to give out at your established business location, but the items didn't actually come from you. They were shipped from various warehouses around the world, often with your return address attached to the items when shipped, as though you [your company] actually sent them. They would include another catalog with the order, with your return address, for an additional fee from you.
Essentially they were using your clientele to sell to. In addition, many mail-order businesses would buy mailing lists, curated by constant maintenance, aimed at very specific sales interests. Now our cookies on our browser provide this info to your search engine/browser/etc. You would purchase the list for a limited mailing, they would include an address in it that would allow them to track how often the list was used. These lists were several hundred USD $100-500 depending on your target market, for 3-4 mailings only.
These lists were gold for the direct mail marketing campaign, regardless of what you sold, product or services.
Sorry for the wall-O-text but the study of how marketing has changed since the advent of the 'net is fascinating and provides insights into what works and what doesn't. The techniques used today by the 'net are/were the very same as used in a modified format, years ago for real-time mail order and other business marketing.
* Sorry, there is a Real Dr. So and so but I couldn't recall his name, my wife once worked for them for a short time, so Nimrod it is...