Here is a deal that sounds too good to be true.

Between now and Aug. 31, Gazelle has instituted a price-lock guarantee for those customers who want to trade-in their old iPhones. The deal is equally good since you don’t have to turn in your handset until after you receive Apple’s next handset, which is likely to arrive in U.S. stores on Sept. 21.



Here is how it works:

Between now and the end of the month, lock-in a quote for your used iPhone on the Gazelle website. Then, keep it until you buy Apple’s sixth-generation iPhone. The price that is quoted will be good through Oct. 1.

Sample prices include: $320 for a “flawless” AT&T iPhone 4S, 32GB, and $300 for a “good” AT&T iPhone 4S, 32GB. Similar models from Verizon Wireless and Sprint, sell for $300 and $280, respectively.

According to CNET, this is the first time Gazelle has implemented a price-lock guarantee for iPhone sales.

This deal is being offered for the iPhone 4S/4, iPhone 3GS/3G, and original iPhone.