I'd sell it. Maybe they go up from here but how much? Sure there may be a number of Metallica fans who would be willing to pay this or more, but how many could actually afford it? What about if the economy gets much worse? I doubt it's a situation where if you sell it for 13k now, you will be kicking yourself in a few years because you could have gotten twice as much or something crazy like that. $13k is already pretty crazy.
I'd sell it especially because it looks super clean and if you actually play them, explorers are easier than the average guitar to bang up, so knowing its value I'd end up not wanting to ever play it unless I decided firmly that I never wanted to sell it.