In the last week, the price quoted by NextWorth for a used 16GB iPhone 4S fell by 8%, dropping to $273 today from $300 on Sept. 6.

Gazelle, one of NextWorth's rivals, said today it would pay $264 for the 16GB iPhone 4S, or 5% less than last week's quote of $277.

Both companies are part of an expanding industry that buys used devices, primarily smartphones but also tablets and video game consoles, from consumers, refurbishes the devices, then sells them online, often using eBay and Amazon, or to wholesalers who may ship them overseas.

Prices paid for the same iPhone 4S by other trade-in outlets also dropped, or in some cases held steady.

At Glyde.com, a company founded in 2009 by former eBay executives, today's recommended selling price was $347 for the same 16GB iPhone 4S, down 4% from $362 a week ago.