People have turned to all sorts of interesting ways to make some side cash, from selling things on eBay (a lot start with items they have around the house that they don't use) to freelancing or selling arts and crafts. Sites like Upwork.com let you work on your own time for a bit of side money, so if you have a really strong skill set you can use to help others, that's one way. If you're crafty, you could start selling your wares from home -- tap your network of friends and associates. For example, a friend of mine makes diaper wreaths for baby showers. they're super cute and make great gifts. If you can get the person buying the item to get the materials, or pay in advance, you don't need to shell out much up front, if any.
The seller, whether a solo entrepreneur or large enterprise, is a vendor, merchant, product creator, or retailer with a product to market. The product can be a physical object, like household goods, or a service, like makeup tutorials. Also known as the brand, the seller does not need to be actively involved in the marketing, but they may also be the advertiser and profit from the revenue sharing associated with affiliate marketing.