I know a lot of you who frequent the marketplace are ad hounds like I am so I thought I'd share my experiences with looking for rare or discontinued lenses for sale. 
I like old lenses. What can I say. I also like alternative lenses to use on my cameras. I have some Contax Zeiss N lenses that I use regularly and I'm now getting into looking at old Canon FD mounts to use with Ed Mika conversions. Lastly, I like some of the Canon EF lenses that are no longer made like the 28-70L which is definitely NOT a rare lens.
Let's take that lens as an example. The 28-70L has never dropped a ton in value simply because its replacement was a disappointment in IQ to many. It's a nice lens. I had one that I bought for the going rate at the time (around $700), but I included it in a trade for a hard-to-find lens that I didn't want to let get past me. $700 was and is a fair price for this lens. It is an OLD lens and Canon will no longer repair or even clean this lens (and it is quite the dust pump). Since then, the asking price from Japanese retailers on eBay is a very unreasonable $900-$1000 average asking price and because of that, the used market has crept into the $800 range and above. I'm not willing to pay an appreciating rate on a lens that should be dropping in value so this one is currently off my list. 
It's nice, but it's not THAT nice. 
Let's look at another example: the Voigtlander APO-Lanthar 125mm f/2.5 SL. This lens sold for brand new as I recall for around $650 US dollars. It was short-lived and the reason may have something to do with the fact that it was manufactured by Cosina who also manufactures modern Zeiss lenses and it either became a competitive lens to the Zeiss offerings or Cosina couldn't handle the load once they took on Zeiss. I'm speculating here based on my research of the lens and the timeframe. This became a cult-like lens because of its beautiful macro imagery and limited production. I traded my Zeiss ZE100MP for one of these straight across 2 years ago because at the time they were equal in (used) value. However, my sample came with mechanical issues when I received it so I traded it back. Since then, I see them all the time on eBay listed by Japanese manufacturers for well over $2K. Usually around $2500 for a newish example and I've even seen them listed for $3500 and up.
Interestingly, these lenses do not sell for these prices. The Japanese retailers seem content at letting them sit and listing them over and over for that one fish ready to bite. Because of their asking prices, however, I see examples of the Voigt lens on FM for $2K or more (there's one on there now for 2.2K) and guess what? They don't sell either.
Since these lenses were all manufactured in Japan, it stands to reason that those retailers would have the best access to finding these lenses that were at one time a dime a dozen. In fact, I'm pretty sure many of them can be found at 'bargain' prices compared to what buyers will pay in the US. Now, you might be thinking "What's the big deal? The market still decides." Well sort of. The trend I'm seeing is that these retailers actually do affect the prices of these lenses... just not UP to the point that they are asking (yet). Since the easiest place to find comparable ads for these hard-to-find lenses are eBay listings by these Japanese retailers, the used market uses these inflated prices as a starting point for a personal sale, for example. They then lower it to the point that it will sell. With the Voigt example above, from what I see selling there, the market budges around $1800 and even then, you have to find the right buyer since a reasonable inflated price for that lens remains around $1500, just like a reasonable inflated price for the 28-70L should be around $700. By reasonable, I'm talking about setting a price where the lens will sell in a nominal timeframe (if not quickly). If you are thinking that these are bargain prices, well, I rest my case. You've been duped by the inflated used market.
I'm certainly not whining. This post is just an observation to elicit some interesting dialog on the used lens market. And I know this is nothing new. Since its inception, eBay has become the swap meet for lazy people with deep pockets and has made pretty much anything that is HARD-TO-FIND also VERY VALUABLE. 2 things that didn't necessarily go hand in hand before, and this is certainly the case today with old lenses.
On the other hand, another tip from my experiences with searches on eBay and comparing those prices to alternative lenses that are still in production tell a different story. Zeiss ZE lenses (current production for Canon mount) can be bought for great deals on eBay. Most sellers have high reserves or Buy-It-Now prices equal to typical used prices found here and FM. However, if you can find a Zeiss ZE lens starting low with no reserve, they can be had for great prices. I got my ZE50MP for a little over $800 in like-new condition. That lens typically sells for around $1K asking prices used. That is a trend I see more often than not. Of course, you will not be competing against the same Japanese eBay retailers for these current production lenses. Most of these great deals are personal ad listings (people like you and me).
I hope this post is interesting to some and elicits some dialog on the used lens market. Because that's the point, after all. I also hope it helps someone who might want to get into shooting with alternative lenses.
