I was in the formerly stop bath stained darkroom and found my old Sigma 150-500mm f/5-6.3 Auto Focus APO DG OS HSM.
It has been discontinued some time ago by Sigma but I see them all the time used for $500 and even less. If you find a good one, it will be very good. They were a thousand dollars when they were new. I used one on my 7D and got some really nice shots.
I will warn you the copy I currently have is the third one. The first two were not worth keeping but this one is a winner. Even Sigma couldn't fix one of them. If you didn't want to use IS, which Sigma calls OS, it was fine optically. The IS would jump so bad it would remove the subject from the viewfinder. I sold if for a hundred bucks with the disclaimer the IS was junk.
Tamron also had a similar lens the Tamron 200-500mm f/5-6.3 SP AF Di LD (IF) Lens. Not as good IQ but it doesn't have IS so that is not a problem! Both or either of these could be stop gap lenses until you can get a really nice lens.
"Even learning everything possible, theres still more to learn on it."
Without actually having the software to try and play with it will be difficult to impossible to learn PS. It is just too intricate. Oh, yeah, you could open and do a brightness or contrast edit. But yo udon't need PS for that. The two most important features to learn are "curves" and "layers". I don't simply mean you know how to click on it but really apply it to a photo. Curves opens up luminosity and color setting and layers opens up masks and selections. All extremely powerful controls.
I sell a lot of used gear mostly lenses but I don't buy a lot of used gear so I am not a good source for where to buy. I have never used ebay to sell anything but I have bought some stuff, parts, etc. and it was OK. I use Craiglist to sell stuff and most best are friends. I did buy a lens from KEH a long while ago. It was better than advertised but it wasn't cheap. B&H and Adorama, Roberts are top retailers and stand behind their stuff. I would think any of them would be fine.
The examples I suggested were found on ebay so if you are not comfortable with buying there, I think finding a good lens in that type is going to be more difficult. The couple things I have bought off ebay have been exactly as described and were fine.
But you must do as you feel is right and comfortable with.