Over the last several weeks, more and more iPhone owners are cashing in their old iPhones in preparation for the big iPhone 5 upgrade. Apple is rumored to officially announce details and release date for the iPhone 5 on Tuesday, Oct. 4.

Gazelle.com, a trade-in site that pays individuals for gadgets based on their condition, reported an 85 percent increase in people looking to sell off their old iPhones, and according to Gazelle's chief gadget officer Anthony Scarsella, the company is on pace to buy back 2,500 iPhones this week.

Compared to last year, when Gazelle bought back about 100 iPhone 3s each day preceding the announcement of the iPhone 4, Gazelle is now buying back more than 300 iPhones per day.

This leap in trade-in traffic is highly unusual, considering that the new iPhone model is not yet available. Apple won't even acknowledge the phone's existence, yet people are still selling off their iPhones as if they were HP TouchPads.

It's really unprecedented behavior, Scarsella says. Usually gadget owners, especially with phones, they wait [to sell back] until the new one is announced.

Another trade-in platform Nextworth is currently offering $210 for a 16 GB AT&T iPhone 4, compared to $235 through Gazelle. eBay Instant Sale, which typically offers the best trade-in values on used iPhones, is currently offering $261 for a 16 GB iPhone 4.

Gazelle expects a surge in trade-ins closer to the announcement, but currently, sales for the 15-month old iPhone 4 remain surprisingly strong.

Our checks indicated strong sales of the iPhone 4, as it remained the top selling smartphone at AT&T and Verizon despite increasing consumer expectations for the iPhone 5 launch, said Canaccord Genuity analyst T. Michael Walkley. During the holiday season, we anticipate strong sales of the iPhone 5, as we anticipate a strong global launch with increased distribution to new carriers including Sprint in the U.S. market.

Last year, Gazelle bought back 3,000 iPhone 3s in a one-hour window following the introduction of the iPhone 4. By the end of the day, that number jumped up to 10,000.