It won't work dude. I went through the same thing trying to buy REZ, they too had 4 copies and the display box (which was perfect). They told me they can't sell rentals and to check back often to see if it goes up for sale. I did, after 3 weeks, they sent two of the copies to Gamecrazy in the mall. I went and bought 1 of them. I needed the original box though not the crappy Hollywood video box, so after I bought it from GC, I went across the way to Hollywood Video, grabbed the display box, went up and asked the girl if I could switch out cases, after I showed her my purchase from GC, she said okay after a bit of hesitation, I asked for the instructions and she threw them in the box as well.
Moral of the story, they won't sell you rental copies. I tried every angle, and they said there was no way due to their inventory/computer or some such. Good luck though.