Hey man, to be honest. That depends.
"One man's trash is another man's come up"
With that beautiful phrase in mind, you must realise the following few things:
If it's a Shiny you obtained by legitimate means (i.e. pure chance), then you might want to hang on to it, for sentimental reasons. In a game plagued by AR codes and hacks, it's good to know there's a legit shiny with you, a symbol of the luck the Universe has so graciously bestowed upon you (I have a Shiny Doduo that I'd keep till ... can't come up with something original, so think of something drastic here. Thanks.)
If it's not yours and you don't think you like it all that much, think: Is there some Pokemon that's not rare, but you still can't get? Like, to complete the Dex, or something like that. Those are the ones who are probably worth more to you.
Is there a (version exclusive) legendary Pokemon you want, and can't get? One whose non-Shiny you're willing to trade for your Shiny? That seems like a good bet. Many would trade a rare / legendary (non-shiny) for a non trained Shiny Pokemon
Or, considering Orange Gaimer's great suggestion: You can EV train it, and touch trade to get a Machamp and then put that up for better offers (think: Shiny Legendary mons, or other trained Battle Ready shiny mons). If you don't want to that, you'll have to settle for what I described earlier.
Again, it's all relative to what you want. Got a favourite unobtainable in-game? Ask for that (even if it's, heavens forbid, a Bidoof or a Patrat. Go ahead, I won't judge. Much.).
Another Shiny Pokemon is also something decent to consider, but it also must be a base Pokemon like Machop, not something like a, for example, a Shiny Gengar or a shiny legendary. You must be fair, otherwise, people will just skip over your what you have to offer.
That about covers the "basics". You must put yourself in another trainer's shoes and think: What would I offer for a Shiny Machop? In a way, it's not easy, because everybody has different priorities, but an average, non-hardcore player would give a fair few Pokemon for a Shiny, specially if they're someone not lucky enough to have gotten a Shiny till now.
Good luck on the GTS! :D