Well, if it helps you any at all - that sale of a 24" SB with the extras would in fact been an aberration and one that would not be expected normally. Some people just want to get rid of something and are willing to "dump" it for significantly cheaper to do so. However I will say that in all my years on this forum and others for cutters and software - I have seldom seen that great of a deal on a used machine so I wouldn't expect to find one but every great while. Most people realize the investment they have made, the quality of the machine and are willing to wait on a good resale price before departing with the machine.
The reasons they came out with the SB are numerous, but they had nothing what so ever to do with the Cougar not being an excellent cutting machine. The primary differences in the two can best be answered by Sherri at That's Scrap or Dawn
Thyme or Jo the dealers for the machines but I think the primary differences are some speed and pressure settings perhaps (more of them I think.) I don't recall exactly and since I didn't "upgrade" - know that I didn't consider them a "huge" difference (if they had been - I would have jumped to the newer machine.)
I don't think you will find anyone who had a Cougar and chose to update to the SB who will tell you that they did so because they were unhappy with the machine in any way. They just wanted the "newest"! LOL I often suffer from that affliction as well! LOL I typically want to get the newest tech tools when they are announced (and usually can if I want it). However, for once - I kept my head about me and stuck with the Cougar because it was a great machine and the SB didn't really offer me that much more and I felt that my Cougar still did everything I needed it to do. (Now, that's not to say that I didn't consider jumping on that $600 24" SB when it was offered up because I knew it was a steal of a deal that didn't normally happen! LOL However, I chose not to again because my Cougar was still a fine machine.) I tell you that to show you that the Cougar is a good option - especially if someone is looking to save $ and get into this level of cutters. So, if you want to get a 24" machine for a "steal" of a deal - your best best is probably going to be a used Cougar. If you are willing to "wait" for a while - you might find a deal on a used SB - but more likely a smaller one than a 24" one as some people with smaller tend to upgrade later to a bigger machine.
I was just looking to offer you some "viable" ideas that will let you get into the cutting and crafting sooner if that is what you are looking to do. Hopefully the right deal will come along for you soon. Only you know your budget and your needs - so check out the forums regularly because any "steal" of a deal will go quickly. Good luck.