Keep in mind that with the advent of denoise AI software, smaller sensors such as on Micro 4/3 cameras or on the Sony RX10V mentioned above will give more than acceptable results at the higher ISO's often required for wildlife photography.
If you still have it, I would be interested.
Using software is the cheapest route to go. DXO Viewpoint is fairly easy to use, and it includes correction for perpective issues in the corners and edges. Tilt-shift lenses are far more expensive. I have old manual focus ones from the old film days: Nikkor PC28mm shift lens (around $400 on eBay these days), and the Canon TS35mm which runs around $800 on eBay. So unless you are making huge prints for fussy customers, software works just fine, and I believe you can get DXO Viewpoint version 3 for around $100.
Keep in mind that the zebras are set up for jpg's and video. If you shoot RAW, you will have to do some testing.
That is a good price, but I can't afford that right now. Anyway good luck on your sale.
Would you be interested in selling the 75mm f1.8 separately, and what would you want for the it?
You might want to check out Matt Osbourne on YouTube. He has lots of information on alternative lenses for Leica.
Also beware of cheap screw mount (LTM) to M adapters, because some of them don't align the lens properly, which means you may not be able to focus some lenses using the rangefinder.
They could be rockets launching satellites. I caught a bunch of these last summer when trying to get a shot of the comet.
Using that kind of adapter would be the easiest route to go. You didn't mention whether that adapter has optical glass in it. As everyone knows, using any kind of adapter with its own optics will cause some degradation to the image quality. Some adapters may be better than others, and the slight loss in IQ may be acceptable. Anyway, I think that we have established that this lens can be adapted to Canon DSLR bodies, and to any mirrorless camera body that takes interchangeble lenses.
The pictures of the lens seem to indicate that it is excellent condition, and the price is a fair price.
I did post some images with my copy of the lens on an Olympus EM-1 body some years ago, but I don't know how to direct anyone to where they are. The images were posted on 10/20/2014, under the title, "Wily Coyote gets his Bird".
If you mean extension tubes for close-up photography, that is possible. Or do you mean something like a 1.4x or 2x tele-converter? By the way the adapters with optical glass are also 1.25x tele-converters as well.
I just tried my copy of the Sigma lens with an old Canon FD 1.4x tele-converter and it works just fine. A 2x would also work. The trick would be finding old Canon FD tele-converters, but then you still have the problem of connecting it to your Canon 7D.
Actually there are adapters, but you may not like the results that you get. Canon made one years ago that had a glass optical element that allowed focus to infinity with the FD lenses on the new EOS cameras. Those are pretty rare these days and expensive. There are cheap Chinese nock-offs, but the optical quality is probably not very good. There are also some simple adapters without any optics, but those will only allow you to focus up close and not to infinity. There was a camera tech who modified FD lens mounts to fit on the newer Canon AF bodies, but it was an expensive process and usually was only done on expensive "L" glass. It would not be worth doing with the Sigma lens. Actually, it would be cheaper to buy a used mirrorless camera, if you wanted to use a lens like that one with a simple adapter.
He did say that it was manual focus, which makes it an "FD" lens. I have one just like it. With an adapter you can use it on any mirrorless camera body. This is the "APO" version that Sigma made. Still fairly sharp when stopped down to f8 or f11, and color fringing is fairly well controlled. There were some problems with the earlier versions, such as haze forming on the cemented inner elements, and the rubber on the focusing ring getting sticky. The later ones don't have any of those problems. New ones sold for about $400 years ago. I think I got mine in the late 80's. The lens is very light and compact for a 400mm, although it is only an f5.6 wide open.
Another option that hasn't been mentioned is "Affinity". It's similar to Photoshop and only costs about $50 with free updates. I'm still using an old version of Lightroom, but have little use for Photoshop, so I wish that Adobe would put out a lower cost subscription plan just for Lightroom. Yes, I agree that $9.95 per month doesn't seem like much, but it does add up over time.
And there is always the issue of getting a new computer, and finding that your software won't work with the new OS.
I acquired a used iMac recently that had the newest Monterey OS on it. Fortunately, it played nice with my old 5.7 version of Lightroom and my Affinity software, but it didn't like my old version of PSE.
Anyway, the information provided here is very helpful, and it looks like I'll be doing the $9.95 subscription soon.