I personally do not know much about survival/skyblock/islands and the /sell features that they possess. I would try maybe /sell, with the desired item in your hand, and then the amount you would like to sell, and then the price. So if you wanted to sell 64 coal for $500, maybe get a stack of coal into your hand, and try /sell coal 64 500. If not that, I'm really not sure myself.
However, If you are trying to receive help in game my first step would always be to use /helpop and assuming any staff are online, you can check with /staff beforehand if you'd like (staff are nearly always on), I'm sure that they'd be more than willing to assist you and answer your questions. If it is beyond staff's abilities to fix it in game, making a forums post is a wise idea, so kudos to you, I'm sure help from a more experienced player is on it's way, good luck!