When does checking receipts at the door become illegal search?

Shopping at a electronics chain-store (screw it, I'm pissed at them, so no sense in hiding their identity...it was Circuit City) the other day, I buy a cheap sound card and speakers. The cashier tells me "keep your receipt handy, they're going to check it at the door". So I ask him, "at what point does checking receipts at the door become illegal search?". He, being a cashier at an electronics store, gives me the complimentary "deer in the headlights" look that is given to anyone asking anything out of the ordinary. And even some of the ordinary ones too. He stammers and hemmms and haws and finally says "uhh, I really don't know, that's probably something better asked of the store manager". At this point, I fail you all, my gentile readers, because I don't ask to talk to the manager. I really should have asked him this question.

As I'm leaving the store, just as door-goon-guy is asking to look at my receipt (and as I keep my purchases on the opposite side of my body from him so he really has no idea what's in my bag...for all he knows I'm walking out of there with a baby llama), I ask him the same question: "when does checking my receipt become illegal search"? His answer was better, but in hindsight, still not good. He said "I think it would be illegal search if store employees wanted to look in the personal bag/purse that belonged to a customer. All we do is look at the merchandise that comes from our store". Unfortunately, I accept this without too much further inquisition (basking in the small victory that while he looked at my receipt, my baby llama is safely hidden in the bag at which he never looked).

Had I been 'on my game', my brain would have given me these follow-up items which should have been mentioned:

After I've paid for it, isn't it my merchandise? The store no longer has any claim to it. What if I choose to put it in my backpack instead of a store bag? What if I decided to put it in my underwear? What if I decide to unwrap it at the cash register and put it on the ground and smash it? If I've already paid for it, that's totally within my rights. It's not the store's merchandise anymore. It's mine. And it is (or it should be) illegal for them to go through my stuff without probably cause. And that leads me back to my original ascertain of "illegal search". I think I may have to go back to that store this weekend and buy something small and test out my theories. Probably need to do some internet reading on the subject before I go picking fights.

This policy of checking receipts at the door after a customer has bough items is a policy I've always, always been leery of. Ever since the days I worked at Price Club (now Costco) and was one of the door-goons checking receipts. I checked daily through the entire Christmas season (October through January). Hundreds, if not thousands of people every day. And to my knowledge neither I nor any of my door-goon-coworkers ever found anybody trying to wander out the front door with a TV hidden among his bulk paper-towels and 16 gallon pickle jars. None. Never.

One day, maybe every customer who shops in a 'illegal search' store will tire of being treated like a criminal for simply purchasing their merchandise and trying to take it home.