Joe,
Welcome to my world!!! I know just what you mean. For me it is all about the tinkering and improving but once that's all done - what next? Shoot great groups and never miss anything - BORING! I've spent around two years customizing my HW100, had it apart more times than I care to think, made new bits with better tolerances including a lovely shinny brass pellet probe (I love a rifle with at bit of bling) and tuned it to within an inch of its life - 4 fps ES over 40 shots. I've tried just about every pellet ever made and pulled out plenty of hair when a new batch of pellets won't group like the one before, but then find a good pellet combo and it is back to a tack driver again. And obviously I have stressed over the ubiquitous to polish the barrel or not - of course being a tinkerer I did polish the barrel! My next "project" is DIY night vision - just waiting on lots of bits to arrive from China. And after that project who knows? Maybe I will build a robot human to shoot for me! Just kidding.
Would I ever sell my HW100? Nah probably not but I feel your pain. My advice to you Joe - DON'T SELL IT! You'll be crying into your cereal the next morning after it is gone.
All the best,
Chris in Oz