One thing that comes to mind is spider webs.
On my Celestron C6S, if you remove the three screws on the secondary mirror you are in deep yogurt as the primary will fall into the scope with a high probability of damaging the primary mirror. My Meade SCTs are different. Another approach is to leave the secondary alone and simply remove the corrector plate with the secondary attached. Before doing so, mark the position of the corrector plate in relation to the telescope tube as it is aligned with the primary mirror.
Misaligned SCT optics.
I've had two run-ins with SCTs with misaligned optics, both were Meades. In neither case did I see anything like the "dark branch" you are describing. However, thinking about it, it could be that the optics are so misaligned that you are seeing the edge of a mirror holder but ...... ?
Misaligned optics with my SCTs produced problems with image sharpness, light halos, ghosting, and collimation. The two SCTs I had with alignment problems .... one I purchased new in the 1980s (a true Halley Scope) and the other was a used telescope I salvaged from the scrap heap. Identifying the symptoms was easy, identifying the cause was more difficult, and I went down several false trails. In the end, both telescopes had a problem with the way the corrector plate was mounted at the factory ... one corrector plate was off centered and the other was cocked ... both due to manufacturing flaws and sloppy work. After identifying and fixing the alignment problems, both scopes perform very well today. Their images compete with my two Celestrons.
One fast way to check the gross optical alignment of a SCT is to simply look down the scope's tube from the corrector plate end. Place the tube almost level pointing slightly upwards from horizontal. Stand in front of the scope about a meter away and look directly down the tube with one eye. You may have to adjust the scope's elevation. Now slowly back away keeping the back of the secondary mirror centered in view until you can see the reflection of the secondary in the primary mirror. You may have to adjust the telescope's angle a little to keep everything aligned. At the point where you can just see the reflection of the secondary mirror in the primary mirror, move your head slightly until the back of the secondary is centered in its reflection in the primary. If the scope is at the first stage of collimation, you will see the secondary mirror and its reflection centered in the primary mirror similar to a collimated reflector's star test.
If the secondary and its image are not centered, you can adjust the collimating screws on the SCT to center the image to get a gross collimation ... at least sufficient to see if collimation is the problem. If you can not achieve a gross collimation using this method, misaligned optics are a likely reason. One of my two scopes could be collimated and the other could not so this is not a definitive test.
The Meade telescope (8 inch LX200 OTA) I salvaged from the scrap heap is now my number one viewing telescope. I don't use it for photography as I don't have a suitable mount for it. For photography I primarily use my Celestron scopes. My Halley Scope, a 4 inch Meade 2045LX3, is my true grab and go telescope sitting ready to go at a moment's notice and its OTA also serves duty as a camera scope from time to time.
Whether you should buy the scope or not is a good question. If you are handy, like to tinker, tolerant of taking risks, and can get a price low enough to make the gamble worthwhile, you may want to go for it.
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Celestron NexStar 4 SE
Celestron Advanced Series C6S (XLT), iOptron GOTO Drive
Meade LX200 203mm OTA, SkyWatcher SynScan AZ goto mount
Canon Rebel EOS XS 1000D