There are a number of sources where sellers may get their products. These include both brick and mortar stores such as retail stores, as well as other online marketplaces and wholesalers and the product manufacturers themselves.
More importantly, it depends on how inexpensively and efficiently the seller can get the product they need. There may be any number of suppliers of a particular product, but the best option for the seller is to choose the supplier where they can best get the product, i.e. as cheap as possible, as reliably as possible and as quickly as possible.
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