Innovation is a good thing, while, sometimes it means old items become obsolete (exactly not very old) fairly frequently.

Gadget recyclers Gazelle.com, NextWorth and eBay have reminded people to sell their old iPhone for cash.

People just put stuff in a drawer without realizing it retains significant value, Jeff Trachsel, NextWorth chief marketing officer, told the Bostonherald.com.

NextWorth offers $250 for AT&T iPhone 4 in good condition, powers on and fully functional, with no cracked display. It has a 21-day period for customers to lock in their iPhone.

Gazelle.com says Sell your iPhone. It's simple on its Web site. It is very easy to sell iPhone with an app, and customers can send in their phone within 30 days.

Gazelle.com offers $284 for the 32GB iPhone 4 in flawless condition, and $264 for the 16GB version.

According to Anthony Scarsella, chief gadget officer of Gazelle, 4,500 mobile phones have been traded in so far during the last two weeks. While, eBay has nearly 3870 available iPhone 4, and majority of them are from sellers.

Meanwhile, wireless operators Sprint and Verizon may give customers an option to switch other smartphone to the iPhone.

BlackBerry trade-ins has increased by 49 percent, and Google's Android operating system smartphone had a 102 percent increase for trade-ins for phones in the last three weeks, Scarsella said to the Bostonherald.com.

The best time to trade in is from now until the day the new iPhone goes on sale, Scarsella added. Although prices may stabilize or even rise then, once the new iPhone is out, prices will quickly drop.