It's Black Friday. You show up at Best Buy to buy that TV that's on special for $199. There's one left, you buy it. Receipt is in your hand, and you head towards the door.
Then, an employee stops you, grabs your receipt and says "Sorry, we are going to refund you the purchase. That guy in line just offered us $299 for it, and we are going to sell it to him".
The TV is YOURS at this point. It's not their TV to do this with. You paid for it. The only difference is, you feel that since it was still "in your possession" you have the right to do this to him.
How do you not understand this?