The way I do it is, for the first four, I take the lowerest buy price at the moment, times it by however many of each I have - I sell them in bulk, after all - divide that by two, and whatever's left is the price I ask for.
As for Formulas, I go for a bit lower than what the lowest price for said item already is.
And outfits, because I mostly sell of things you can't get in stores, I go by what it says the outfit can most likely get. Of course, since that can change, it might not seem like that.
As for clothes in general, if it's something that can be got in the store, I won't sell it for anything over what you can buy it for in a store. However, if I can get more by selling it outside of the market than selling it here, then I will do that. For those that can't be got in a store, I sell the item for lower than what the lowest buying price asked at the moment is.