The price of second-hand iPhones had been tumbling for weeks ahead of the expected 9 September launch of Apple Inc's new  iPhone 6s and 6s Plus. Old phones drop an average of 20% in the month leading up to the launch of a new model, according to used gadget online retailer Gazelle.

Values are not likely to pick up. NextWorth, a buyer and seller of second-hand electronics, tracked eBay sales around the September, 2014, introduction of the iPhone 6 and 6 Plus. It found the price of an iPhone 5 dropped by about 35% between July and December.

"It’s not just the most recent iPhones that are worth proportionately less,” said NextWorth’s chief marketing officer Jeff Trachsel at the time. "Values on older models tend to fall faster.”

In fact, it’s becoming difficult to find a quoted price for an iPhone 4 or anything older. These models will not run the latest version of the Apple operating system, iOS 9, due to be available for other devices in fall, 2015.

Still, it’s not all bad news if you plan to upgrade to an iPhone 6S. The sheer number of trade-ins creates a massive business opportunity for auction sites and other sellers of used gadgets. The day before the planned launch, Gazelle was handling four iPhone offers a second. NextWorth says volumes are up 74% over last year's iPhone 6 launch.

That means plenty of businesses want to help you sell your old iPhone (or buy one) and they make it as easy for you as possible. For one, they allow prospective sellers to see how much they’ll get for a pre-owned phone with only a few clicks. If you’re selling, it pays to shop around to find the best combination of price and convenience.

The differences in offers can be massive. The day of the purported 9 September announcement of the iPhone 6s, uSell was offering $320.38 for an unlocked 16GB iPhone 6 in good condition. Ebay estimated the same model would fetch $432 at auction.

Whatever trade-in method you use, prices are dependent on model, condition and carrier. Factory unlocked phones are the most valuable.

Bear in mind that "condition” can be subjective and you may end up being offered less — or possibly more — for your phone once the buyer has actually seen it. If the offer is revised, remember you do not have to accept it and you can ask for your old iPhone to be returned.

Don’t forget to check whether quotes include post and packing. Some services send a prepaid shipping kit to your door, so you can sell without leaving the house. Others leave you to do everything from drawing up advertisements to standing in line at the post office.

As a rule of thumb: Less hassle for you means a lower price from the buyer.

Where to sell your iPhone

Amazon.com: Offers consistently among the highest prices for trade-ins.

The catch: You have to take your payment in the form of a gift card rather than cash.

Ebay: Provides potentially among the best returns and now offers a simplified way to see how much you could expect to receive for your iPhone.

The catch: Getting the highest price could leave you without a smartphone while you wait for the iPhone 6 to ship.

Craigslist: Has no transaction costs. It’s just you and the buyer. In theory that should give you the best possible price.

The catch: You do the haggling, arrange for the exchange and take all of the risks.

uSell and Glyde: Act as middlemen linking you directly with a buyer.

The catch: They are less hassle than eBay, but you could come away with a little less cash.

Gazelle and Nextworth: Buy your used iPhone directly from you. Simple.

The catch: You won’t get the best possible price.

Many of these outlets except Craigslist and eBay offer some form of price lock-in which means you can agree how much you’ll get for used iPhone now and then send it off later. That means you won’t be left without a phone and you can change your mind if you decide not to upgrade.

The winners: Those with very old phones. Some people will be happy to see used iPhones falling in price — because it’s an upgrade for them from an even older model. What’s more, wireless carriers and other retailers are also likely to offer the last model for much less than it did just a week ago. Retailers will be clearing inventory and there are bargains around for those who don’t have to have the latest iPhone model.

"If you aren't too bothered about having absolutely the latest technology then it’s definitely worth waiting a few weeks to get a low-cost [older model of] iPhone,” said Gazelle’s senior tech analyst Alyssa Voorhis in a September, 2014, interview.

(BBC)