Hey guys, my under tank petcock switch thing has crapped out on me and leaks gas from the internals and it runs down the outside outside of the tubes the fuel lines connect to. Ebay has much better prices on tank valve from '04+ GS500s.... Is there a a difference at all in the parts, or are they the same valve? By pictures, they appear 100% identical, but I don't want to waste money on something I can't use

All years the same.
Most likely you can easily fix it. There is a o ring in it. That will crack and dry out because that valve is rarely turned off and on. So when turned off after 4 years, the o ring can break.
Lift off the spring clip and check the o ring. Replace it and probably you will be OK.

I've already taken the thing off and checked it out. I don't think it's the 0-ring just because it does not leak from around the ring. The gas  has a steady dripping when the valve is on and the lines are connected. But it's 100% dry where it meets the tank. And with the tank OFF, it still seeps out



It leaks where the longer tube meets up with the body of the valve. It seems like I need to pull the tube, and replace an O-ring on it then re-install. That's the best way I can describe it, but I don't think that's possible

I was not referring to the o-ring at the tank.
Remove the clip that has FUEL OFF ON. Lift out the little valve thingy. There is a small o ring on that.
Never nurts to get a replacement. Until you get one, check that other o-ring.

Excuse my ignorance! I had no idea there was an o-ring there. Time to go check it out 

The o-ring behind the turning part of the valve is still soft and pliable... But just taking it out and putting it back in, there is now no gas smell from the valve at all. so that strikes me as a very good thing

Well, it's a terrible picture, but this is what my o-ring and what not looked like when I pulled it off

Could be it was dirty or stuck. Taking it out mat have freed it up.
Go with it. Order a new o-ring. Replace it when you feel like it or if the problem comes up again.

Thanks for the help! I never would have thought to look there. I thought the ON OFF shiny piece was just a trim piece, not an integral part of the assembly