I buy my sigmas used. Even though they are cheaper to start, they take a big hit in price with the first owner. A new Canon or Nikon will not loos much in value just off the showroom floor, so I buy those new. (when i can afford it)
You are right!
I've also found more problems with some brands of used 3rd party lenses (except Tokina), and I've bought lots of old lenses. I keep an eye on our local Craigslist, and when I see a high end lens selling for too little, I buy it and use it until I decide if I am going to keep it permanently, or sell it when another good buy comes along. I do not lose money doing this, and I get to test and use dozens of lenses long enough to learn their strengths and weaknesses. My experiences with the lenses is usually pretty well identical to the lens reviews, but I like to see for myself.
Something new just today, I just brought a 250mm f/5.6 Zeiss for Hassleblad lens ($200). Now, I'm waiting for a adapter to arrive so I can compare it to my other lenses. I ordered M42 and Olympus OM adapters as well, I have lots of those lenses around. I want to try my 28mm OM on my Canon bodies as well as some of the old M42 lenses which are not popular brands, but should be good ones.
Maybe I'll put up a page on my web site to compare them. It takes a lot of care to do a good job with MF lenses, so its something I have to wait to find time to do it.