Upload & Sell: On
Thanks for the plug. The Lens Price Database, Lens$db, has been publiished three or four times per year, since 2006.
I think Will (trenchmonkey) is right, it depends on individual circumstances.
I've attached example price history graphs for five lenses, from March 2006 through January 2013.
You can read the tea leaves any way you want to, but it's clear that there's no one trend that fits all cases. The values of Canon lenses were greatly affected by the tsunami that hit Japan in March 2011. No wonder, it was a terrible natural disaster that had profound effects on all aspects of life in Japan.
After the direct effects of the tsunami began to fade, subsequent used lens prices either fell or continued to rise, depending on the lens. I think the reason that the prices of the "Mk I" versions of the 300/2.8L IS and 500/4L IS continued to rise is because their replacement Mk II lenses are so expensive, and the Mk I versions have stellar IQ. OTOH, the 50/1.4 and 70-200/2.8L IS (Mk I) prices returned to more-or-less pre-tsunami levels. The 50/1.4 has no Mk II version, but the 70-200/2.8L IS II does, and it's quite expensive compared to the Mk I, yet its prices fell, too. In the case of the f/2.8 zoom, the Mk II has significant IQ advantages (for some users). and so proportionately more photographers 'upgraded', than for the super-telephotos, which offered the main advantage (IMO) of substantially reduced weight. It's easy to explain this away (and even convince yourself that you might actually be right), but there's no way that I'd hang my hat on the proposal that the behaviour of future prices can be accurately predicted by past prices. For some lenses, there is no "critical price drop" - the prices just keep rising. The longer you wait, the worser it gets...
I think the prices for the EOS 1DsIII, 1DIII, and 1DIV show similar behaviour, in that it would not have been too easy to predict what happened. The prices of the 1DsIII and 1DIII rapidly fell after the introduction of their "replacements" (1DX and 1DIV), yet the 1DIV prices remain relatively strong after the 1DX. I expect that this is because many people like to have a 1.3x CF 1D-series body, and/or they can't afford to replace the 1DIV with a 1DX.
I'll probably release a new version of Lens$db soon, and I'll update the price history graphs at that time to include all of 2013.
Here's some older threads with price history data: