The market for used iPhones is a funny thing.

It hums along steadily most of the year until, just before the launch — or, more accurately, the expected launch — of a new model, things go nuts.

This year, more than ever. A few data points:

  • According to a survey by Hanover Research, an unprecedented 48% of iPhone owners plan to trade up to whatever Apple has up its sleeve.
  • Gazelle, a leading trade-in site, saw iPhone offers peak at five per second one day last week before settling down to two per second, up 50% from last year.
  • Another site, NextWorth, saw average daily iPhone traffic jump 350% from the previous month. “That’s up from a lift of 182% last year, or almost two times the acceleration,” NextWorth’s Jeff Trachsel told Computerworld. “There’s tremendous pent-up demand for a larger iPhone.”

The flood of second-hand iPhones reassured Wall Street and help lift Apple’s shares, which hit new all-time highs every day last week.

The market value of those second-hand iPhones, however, has been sliding the other way, according to eBay, where nearly 400,000 iPhones changed hands in the two weeks before the last three launches. Historically, iPhone prices on eBay peak three weeks pre-launch.

Several re-sale sites, including Gazelle and NextWorth, offer to lock in prices before they fall any further (see attached chart).

Both companies have timed their offers to Apple’s rumored (but still unconfirmed) Sept. 9 introduction and an estimated Sept. 19 launch date.

Gazelle will be buying used iPhones right to the bitter end. If you pass by Apple’s Fifth Ave. store on an iPhone launch day, you’ll see them working the lines.

But if you wait that long to sell, you’ll get whatever the market will bear.

Which site gives the best deal? The Wall Street Journals Rani Molla posted a handy chart Tuesday (see below). The prices shown are the highest offered for iPhones in good condition, chargers included. Phones that are unlocked are more valuable than ones that aren’t, especially on sites like Amazon.

Source: WSJ

 

Follow Philip Elmer-DeWitt on Twitter at @philiped. Read his Apple AAPL coverage at fortune.com/ped or subscribe via his RSS feed.