A reseller is a company or individual (merchant) that purchases goods or services with the intention of selling them rather than consuming or using them. This is usually done for profit (but could be resold at a loss). One example can be found in the industry of telecommunications, where companies buy excess amounts of transmission capacity or call time from other carriers and resell it to smaller carriers.
According to the Institute for Partner Education & Development, a resellers' product fulfillment-based business model includes a corporate reseller, retail, direct market reseller (DMR), and an internet retailer (eTailer); less than 10 percent of its revenue comes from services.
The word has grown in usage and been made popular by the computer industry, who clearly did not understand what retailers do (i.e. more that just 're-sell'). It is thought by some retailers in the UK to be an insult.