What would you do in this situation?

[Panic]

I used to consider this girl my friend. We're not close but shes very nice and used to hang out at lunch sometimes in high school. Well recently my sister bought some tickets to a midnight premiere of a movie a few days in advanced but then realized she couldn't go. I put it up on my facebook status that I had 3 tickets to the movie that I wanted to sell, as my facebook only has 80 friends all of whom I know in real life, i wasn't selling them to some stranger or something. Anyways this girl buys them then comes over and picks up the tickets. When she gets there she says she didn't have the $28 for the tickets and could pay me back the next day. I trusted her, and I didn't have the heart to tell her no, so I handed over the tickets. Well now it's been over 3 months of politely asking this girl for the money she hasn't paid me. I have given her multiple options even sent her a money order from paypal and she has not paid. I am getting in trouble with my sister since the money was hers and she needs it back. This girl has forgotten to drop it by my house at least 10 times, and has sat on a Paypal transaction for a week now. I tried to drop by her house but I don't have her address. Does anyone have any suggestions on how to handle this situation maturely? I am getting to the point where I am going to send her a seething email which I know isn't the answer. I thought about putting a deadline to pressure her to get her act together but I've done more subtle dead lines that haven't helped.
Any ideas?
In short I will never sell something to someone again who doesn't come up with the money up front. It's a shame that you can't trust some people. Its like she expects me to just forget it and give her the tickets. If I do that, then she'll just expect everyone one to do the same, which I feel is wrong.
Thanks!