You’ll want to be careful with stuff like this, because you have to hope that you break even. Remember that Shimmering Dust MINUS trading post fees has to be worth more than the (Glittering Dust -TP fees) + the stuff that you needed to feed the Mystic Forge.
Right now, you’re much better off farming dragons or meta events with end-chest for in-game coin due to guaranteed rare drops:
http://guildwarstemple.com/dragontimer/
If you’re not high level enough to get rares that sell decently from those event rewards, then you’re likely also in a situation where four skill points would be better spent on slot skills than on the MF.