Was looking on ebay and craigslist for After Effects.
I saw AE CS3 supposedly sealed in retail packaging for around $500.
Looking on Craigslist, I found a couple of computers that had the Adobe Master Collection on them.
Is this too good to be true, should I assume that the software has been pirated and or illegally copied or something?
It's probably safe to assume, yeah. Many sellers will load their computers or hard drives up with hundreds of dollars' worth of copied or cracked software to boost apparent resale value. Not like you'd realistically end up in trouble for it anyway, though, even if you did buy a computer pre-loaded with "hot" software.