If you want to sell extras (usually cause you hit max level of the items in question and combining more is pointless):
Go to the p-pad (far right option, the options) and select the first tab (the one that has a diamond in it, top left option on the pad).
You'll see your items.
The L1/R1 tabs will move between items, costumes, and accessories.
Select the costume/accessory that you'd like to sell copies of.
Hit Triangle.
Select which ones you want to sell using Circle.
Hit triangle again, then say yes to the prompt.
If I get that wrong, it's because it might be another button, but you can sell from that screen.