Another way is to use a multidimensional arrays

int yo[10][10];

10 across and 10 down for a total of 100 slots. Now picture the 100 slots on the screen. Each slot with have a number 1 to a million. Each number would represent an item from your inventory. Now set up the positions for the 100 panels. Each panel is actually a button. Set up the function so that it can be clicked, moved and dropped into a new spot.

Yes I know you probably don't need 100 spots but it was an idea. Note that it doesn't have to be a a power of 2 (10 x 10, 5 x 5 or 3x3) it can be anything numbers and the position can be anything, that is up to you.