You can sell them whenever you like, but it's inadvisable to if you're looking for a specific item. Using Super Sword as an example, which needs Ax3 Bx2 and Cx1. Average Potion Kit needs Ax1 Dx4 and Ex4. If you sell them one at a time, you may end up buying the Potion Kit, which resets your A, D and E back to 0 in the bazaar, even if you sold 80 As originally.

I think.