If you want something to show for all your time on Runescape, why are you quitting exactly?
Pouring your money into a skill makes no sense to me personally. If you were really "going to quit", you would have done it by now. If you're still playing for some purpose or another, then it's obvious to me that you don't want to quit in the first place.
It's impossible to have "something to show" for your time on runescape. Think about it. It sounds a little stupid when you say "I wanted to have something to show for my time on Runescape, so I raised my construction level from 50-60" or whatever. Who would you be showing this "achievement" to anyways? If in fact you are quitting, you wouldn't be showing it to anyone on Runescape, because you wouldn't be quitting Runescape if you were. And more likely than not, people who don't play Runescape probably wouldn't give 2 cents if you raised a few levels in a game just before quitting anyways.
I still say you should keep your money/stuff, instead of wasting it on something/giving it away. It's quite likely that you will come back one day. I thought I was gone for good, and guess what, I'm back here again. I thought of giving my stuff away, but I'm glad I didn't because I would have had a hell of a time when I started to play again.
You're probably just ready to take a break. Take a few months off RS, come back, and it might be more fun to play. Don't waste your stuff, you might regret it later.
Gee, that was long. I hope you read it though.