That sux man, I had a cat when I was in middle school who got hit by a truck and had to have a ton of surgery (lost an eye, minor brain damage, and tons of stitches). It cost my parents a ton of money and then a month later he was diagnosed with leukemia and we had to put him down. It sux wasting all that money and then your cat ends up dying. I'm glad you know what happened to him at least. Its better than worrying for months where he wound up.
I just play the current one. I think the only time you should go back is if playing the original impacts your current story (Mass Effect, The Walking Dead, etc). I actually played the entire Metal Gear Solid out of order and still really enjoyed it.
My town doesn't have any of those mythical Gamestops that doesn't hassle you about trade-in deals and preorders. I am not surprised that they're declining though. Amazon has been doing a great job of giving competitive, if not better prices on used games. Also the majority of people who want a console probably have the ones they want now. It'll be interesting to see if their stock picks up when the next generation of consoles hits.