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Want an iPhone 6? Your year-old iPhone could be worth $300

The market for used iPhones has always been hot compared with other mobile devices, but it's gotten even more active since the new iPhone 6 was introduced this week.

The iPhone 6 and iPhone 6 Plus may be costly, but they will retain their value better than most other phones. Apple

Apple's iPhone 6 doesn't come cheap, but the good news is your old iPhone will help you pay for it.

People interested in upgrading to the newest Apple smartphone, which officially goes on sale Friday, may be happy to find that their older iPhones can sell for up to $300 or more and some companies are offering special promotions for the devices.

Among the deals available, Verizon is offering customers a free 16-gigabyte iPhone 6 when they trade in certain older iPhones and re-up their contract for two more years. T-Mobile a few days ago said it will guarantee the best trade-in value for used mobile devices among the major carriers -- find a better price from AT&T, Sprint or Verizon and it will match it and give you $50 on top of that. Also, retailer GameStop will kick off a "Trade-In Weekend" next week that will include higher store-credit rates for certain smartphones, including iPhones and Samsung Galaxy S devices.

The flurry of activity is typical ahead of the launch of a new iPhone, but this year is poised to be particularly frenetic with high demand for the larger iPhone 6 and jumbo-size iPhone 6 Plus. Apple boasted on Friday of record orders overnight after preorders began, and both Apple and its carrier partners are already delaying shipment times. All of that points to a robust market for trading in iPhones.

"I definitely think it's going to be a blockbuster year for iPhone sales and a blockbuster year for iPhone trade-ins," said Alyssa Voorhis, senior tech analyst at electronics trade-in company Gazelle.

There's a focus on iPhones because older models retain their value better than any other major mobile phone out there, according to Gazelle. That gives owners plenty of options. Prices tend to be better for iPhones because of their demand in the used-phone market, which includes overseas customers who can't afford a new iPhone model and US customers interested in buying an iPhone but not a two-year contract. The base price for a new iPhone without a contract is $650.

Glyde, which runs an online marketplace for consumers to buy and sell games and electronics, said as of Friday prices for the 32GB version of the iPhone 5S from Verizon can sell for $334, while the much older iPhone 4 sells for about $64. Trade-in site NextWorth said Friday a 16GB iPhone 5S from AT&T sells for up to $237, with prices for older models fetching from up to $82 to up to $172. Plenty of sites -- including Apple, Gazelle, NextWorth, GameStop (which offers store credit or cash) and T-Mobile -- provide offer prices or estimates for older devices, so customers can easily shop around.

Glyde said it's processing triple the number of iPhone sales offers this September than a year earlier, when the last iPhone launch occurred. It's seen a 50 percent rise in listings of phones using Google's Android operating system in this week compared with last week, indicating potential interest of people looking to make the switch to Apple from a competitor's phone.

Unlike last year's more gradual upgrade to the iPhone 5S, the move to iPhone 6 is more significant. The iPhone 6 has a 4.7-inch display and the 6 Plus has a 5.5-inch display, a big increase over the 4-inch screen on last year's iPhone 5S and 5C. In addition to being slimmer and lighter, both models also include a 64-bit A8 processor with improved graphics, an upgraded 8-megapixel rear camera, better battery life and an NFC chip that allows you to use the phone to make payments.

Since Apple unveiled its new smartphones Tuesday, the 2-year-old iPhone 5 and year-old 5S have dominated trade-ins on Gazelle's website, representing 46 percent and 34 percent for all iPhone trade-ins, the company said Friday.

Glyde Marketing director Joe Senn said his company has noticed a big spike in even older iPhones -- 3- or 4-year-old models -- going on sale on its site, an indication that plenty of Apple customers have been sitting on the sidelines in recent years but are now excited enough about the iPhone 6 to finally make an upgrade.

"I think early indications show a pretty successful launch for Apple," Senn said.