NEW YORK (MainStreet) — If you’re looking to sell off your iPhone 4 to make way for the next generation model, your best bet may be to put it up for sale this weekend, or else wait a few more weeks.

Last year, the trade-in values of the iPhone 3G and 3GS began to drop roughly 10 days before the iPhone 4 was officially announced as rumors of the newest model began to circulate in the media, according to eBay sales data collected by NextWorth, a popular electronics resale website. By the time the iPhone 4 was finally unveiled, the 3G and 3GS prices had dropped by 10% and 15%, respectively.

The resale prices of the two older models continued to fall by another 10% between the time the iPhone 4 was announced and ultimately released, NextWorth’s data shows. The simple reason for this is that when the newest model comes out, too many consumers with older versions get the same idea and try to trade it in.

“The market gets flooded leading up to the announcement and after,” said Jeff Trachsel, chief marketing officer at NextWorth. “It’s a huge time for people to sell their old phones, and with all that extra inventory hitting the secondary market, the price just naturally declines.”

The values of the iPhone 3G and 3GS only began to stabilize and increase again nearly a month after the iPhone 4 came out, but even then, the increase was slight.

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As of now, the rumor is that Apple is set to announce the newest iPhone on Oct. 4, or 12 days from now, with the actual release date expected to come later in the month. If history is any judge then, the window of opportunity to get the best price on the iPhone 4 may close by the end of this weekend.

To put that in perspective, Trachsel says the iPhone 4 is currently selling in the low $200 range on Nextworth, so if it suffers the same 10%-20% drop that older models did last year, it could mean owners end up losing $20-$40 on the resale value by waiting too long. Of course, some simply might not have the luxury to sell sooner.

“Some people have a backup phone or can live without a phone in order to lock in a price early,” Trachsel says. “Others will wait until they have the new phone in hand.”

For those who are interested in selling back your old iPhone, check out MainStreet’s roundup of the best places to trade in the iPhone 4.

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