What is it worth to you to sell it? List it there. If it sells, great, if not, you know you like it more than you'd get selling it.
It will take a very specific buyer. You will broaden the interest level the cheaper you list it as very few people will want a full length pcc. Even fewer how you have it set up, tailored for YOUR wants. Good looking race ready MPX, but that really does limit the potential buyer's pool.
Put the price you think is a little high. You can always come down. It it is a factory build it worth more why I don't know cause there is no magic assembling these rifles. Unless the parts are sub-,standard. You can buy and entry level rifle for 7-800 dollars. However pistol caliber carbines seem to be the rave now. Depending on the brand. Who made the rifle to begin with? I can better asses it's value based on manufacturer.