It's a well-documented fact that every time a new smartphone comes out, whole gaggles of eager-eyed fanboys ditch their 9-month old versions of pretty much the same device to upgrade to the slightly newer, slightly more chamfered version. Just like the rest of life, when to sell is all a question of timing.
Thanks to management consultants' New Favourite Thing, 'big data' (apologies for the terrible buzzword), it's now pretty easy to work out exactly when the best time to sell a device is to maximise revenue, and how the release of a new version effects the price of the old.
To do so, we used the wealth of freely available data provided by eBay (through the medium of analytics service Terapeak). eBay's probably the biggest retailer of used phones, giving an excellent picture of what happens in the market.
Take the release of the new iPhone 5S as a case in point. Following the initial iPhone announcement on the 10th September, sales of the iPhone 5 went through the roof, doubling in value from around £40,000's worth of iPhones sold every day during August, to £85,000 sold after the announcement.
Funnily enough, however, the actual price doesn't change much. A used iPhone 5 was fetching an average of £350 prior to the launch announcement, and has only dropped by about a tenner since. The same is true for the 4S, which has been averaging around £200 since spring.
So, the conclusion is that it doesn't matter if you sell your phone a month before launch or a month after: as long as your listing picture isn't ridiculous, you'll probably still get about the same amount of cash for it.
However, what about over a longer timescale?
Let's look at what the launch of the iPhone 5 did to the prices of the 4S:
As you can see, after the first few months, the price takes a nosedive. This is true for most phones, not just iPhones: this graph shows the price of previous-gen smartphones in the months after the launch of their successors:
As you can see, the price starts decreasing more dramatically around three months after launch.
The conclusion, then, is that you should sell within three months of launch if you want to get the most bang for your second-hand buck. Leave it later, and the price doesn't fall off a cliff, but tends to decrease by about £10 per month, as a rough rule of thumb.
In doing the research for this article, I stumbled across a few other interesting trends. The most surprising is the spike in phone sales every weekend. Intuitively, it makes sense -- people have more time to flog stuff on eBay on the weekends -- but the difference is dramatic, with weekends (especially Sunday) often having double the listings of weekdays.
Most importantly, this increase in sales means lower prices in general. Essentially, you want to be buying on the weekends and selling during the weekday.
One thing eBay probably doesn't want you to know is that on average, stumping up for the added eBay extras like long descriptions or big galleries doesn't do much for improving the amount you get for your handset. No, the only thing that helps (depressingly) is loading your description and headline with keywords. Oh, and having a high seller rating, obviously.
If you want more general tips on where and how to sell electronics (not just smartphones, either), you can also check out this guide on flogging your old junk.