The longer you wait after the release of the new iPhone 6, the less you’re likely to get for your older model. By Cameron Huddleston, Former Online Editor September 9, 2014 Apple just unveiled its new iPhone 6 and iPhone 6 Plus, both of which go on sale Sept. 19. So if you have an older iPhone model you plan to replace, you might want to act quickly for the best chance to get top dollar for it.SEE ALSO: 10 Ways to Make a Smart Phone Pay for Itself Prices on the secondary market for older iPhones already have dropped slightly in anticipation of the new iPhone release, says Mark Bowles, co-founder of ecoATM, which operates more than 1,100 self-serve kiosks in 42 states that allow consumers to recycle their old electronics for cash. Once the new model actually hits stores -- Apple will start taking pre-orders for the iPhone 6 on Sept. 12 -- older models will fetch even less on the secondary market, according to research by uSell.com, an online marketplace for used gadgets. USell.com examined thousands of used iPhone sales on its site following the 2013 iPhone product launch and found that prices on older models started dropping within weeks after the release of the model. Among its findings: -- Two weeks after a new iPhone launch, old iPhones lose about 11% in value. -- Four weeks after launch, old iPhones depreciate about 15%. -- Six weeks after launch, old iPhones depreciate about 18%. -- By week seven, old phones are worth about 21% less. Advertisement So that means that an iPhone 5s in good condition that you can sell for about $260, on average, now at trade-in services such as uSell.com and ecoATM – as well as Gazelle and NextWorth – would likely sell for about $204 in two months. The iPhone 4 will lose more value than other models with the release of the iPhone 6 because it won’t be able to run the new iOS 8 operating system that will be released along with Apple’s newest smart phone, Bowles of ecoATM says. (Apple says the iPhone 4S will be compatible with iOS 8.) The price ecoATM is offering for an iPhone 4 in good condition already has dropped 75% to $52. Typically, when you buy a new iPhone, you can trade in your older model for a credit toward the purchase of the new one. However, many retailers only accept select phones, Bowles says. And most won’t accept damaged phones. But trade-in services such as ecoATM, uSell.com, Gazelle and NextWorth accept a large variety of smart phones and will even pay for damaged phones (see What to Do With a Damaged Smart Phone). Even if your phone is in good condition and you can get a credit, you might get more money by selling it. Take the time to compare offers from trade-in services with the credits retailers are offering. And note that ecoATM’s kiosks pay cash on the spot for used phones, but you’ll have to ship your used phone to an online service and wait for it to be inspected before you receive a payment. Regardless of where you sell your old phone, Bowles says you should turn off the “Find My iPhone” feature (in settings under iCloud), erase all of your data and disconnect that phone from your account so no one has access to your personal information.