I hear ya...haven't purchased a brand new Mac in a long time...but even with my used purchases...I still hate to get rid of the previous (for example) 4 year-old used computer I
was using...when I get a "new to me" used 2 year-old computer.
One thing I like to do when I've decided to get rid of the old computer...is make a "clone" of my old computers hard drive. That way I can always "virtually boot" into my "old computer" if I'm having problems. Of course this assumes that everything on the old computers HD is compatible with the new computer (OS version & apps.)...or an old computer is still around the house that can boot the old computers cloned HD.
A 2nd reason I hate to get rid of an older computer is...if it was capable of booting an older OS version that the new computer is not capable of. For example the new computer may only be capable of booting into OS 10.8 versus an "old computer" being able to boot into 10.6 and/or 10.7. I always have lots of older software lying around...and it's always nice to be able to boot into older OS versions to run the older software.
I'm sure with your experience you may be thinking the same things. Of course getting $600-$800 for the old computer is always nice to offset some of the cost of the new computer!
- Nick