Also, it wouldn't be THAT hard to find some Sythe users.
Right click any of the Runescape pages > view source.
They track which page you're coming from whenever you visit ANY Runescape page. It wouldn't be hard for them to see "Sythe.org" and flag the IP address, then investigate any account that has used that IP address.
It would only take ONCE in all of the time you've ever been on Sythe for you to go from Sythe to Runescape and they'd have your IP address stored for later review.
You gave them the last letters of your username, which would eliminate LOTS of people on that possible list, as well as how much of a cash stack you had along with two items. I highly doubt there are many other accounts that have all of those same characteristics in common.
I'm not saying that this is how you got caught, but it's a possibility.