Welcome Leading Man, nice post! A bit long, but your vivid descriptions kept my eyes dancing along the lines all the way to the end.
Well, I am also new to PU, but before utilizing some of the techniques I've learned I would have similar situations where it seemed like the female was interested, but she seemed to playfully push me away.
First off, kudos for getting out there and approaching. Although it seems simple, this is the most important step to overcoming PU anxiety. Keep it up Bro! Now, as for your interaction...
In my opinion, I think she placed you in the nice guy category. Yea man, I know it hurts, but you must remember, beautiful women, especially the women who work or shop at high-end retail stores (Abercombrie, Hollister, Victoria's Secret, etc...) are being hit on all the time. Imagine a gorgeous women like that, and how many NICE guys like yourself come along everyday playing the innocence gambit and talking themselves up like real hard asses.
For instance, in your post you say that you walked in and she smiled at you, and then you walked out. Dude, 3-second rule man! You've already lowered your value by walking out of the store and then immediately walking back in - you don't think she hasn't seen a hundred other guys do the exact same thing. In her eyes, you were probably just another guy with no confidence, no game, and no fun. Blah!
Also, you let her talk you into trying on clothes. If you weren't going to buy the clothes, don't go try them on! Make her continue showing you around so that you can pump the buying temperature. If she was attracted, it was probably lost whenever you left to try on the clothes. Remember, these first interactions are crucial, don't give her time to think (nor yourself for that matter), be spontaneous (or have several canned routines at your disposal to tailor to the situation). As she continues to show you around the store to different shirts be playful. Tell her, "Wow, your bf must be a real fashion freak! Is that where you get your style from?" Now you're being playful, but you also get to extract some crucial piece of information. If she says, "No," then you can respond, "Well, yea its probably for the best. I don't think I'd want my gf dressing me up in these clothes. Ewwww!" Or, if she was to respond on the affirmative, you can easily make fun by swapping the comment, "Wow, I feel really bad for that guy, if he's got to parade around in such boring clothes." BE CAREFUL, you need to immediately DHV after these or switch to another conversational thread. You don't want her dwelling on the negs you just made.
Lastly, your story seemed incongruent with your personality (at least the portion where you told her it was illegal did). Just from your style of writing and the way you approached you don't seem like the kind of guy that goes around committing crimes, and I bet she could sense it too. Remember, women are far more sentient beings than us men, they can smell bullshit from a mile off. Especially when they're HB9/10. These are even more likely to sniff you out because they have honed this skill over many years of fending off AFCs and pathetic sexual predators. Instead, don't let her run your frame. She asked you if it was illegal, why does it matter. It was a fun story. You should have responded with, "Are you one of those good girl gone bad types. Man! And I saw so much potential [smile, then shake your head]. I'm not into breaking the law unless the benefits outweigh the consequences [stare directly into her eyes]. I'm not stupid! [smile]" Remember you can still be yourself when doing PUA. The way I see PU, is that it is a way of conveying your personality in the most favorable light. Its getting women past the initial appearances and getting them to see just how cool and amazing a person you really are. Always be you. If you can't, what's the point?
Even at the end of the conversation you shouldn't have let her off so easy. Although at this point it was probably past the point of saving. I know she's working and all so you don't want to seem needy, but when she told you maybe she'll give you the number the next time your in the store (a classical cop-out by the way) you could have said, "No, I never shop here. It just looked like you needed something to do. A pet project remember. [Pause and smile for a brief moment, and then walk away]. Now at least if you do come back she doesn't have the lameo impression that you left her with (I say that as a sincere form of criticism, not in offense).
So my fingers are tired, but I'd just like to say congrats on making an approach and posting this field report. Your writing style was so clear I almost thought I was there walking through the store with you. It made commenting much easier.
Keep it up bro! Don't let any bad experiences knock you down. I'm sure everyone on here is going through them. I am just recently getting past the social hook and attraction stage. I'm not even sure exactly how to #-close yet

I'm successful at gaining rapport and then attraction, but just leave the interaction hanging on a thread because I'm too scared to ask for her #. Also, remember that if you want to get the girl, you've got to be prepared to lose her. It's a risk, and I certainly need to follow the letter on some of my own advice in that department.
Cheers