good buys vs. bad buys

We have 2 sites both have good ranking in google but, one of the sites have a few keywords that bounce up and down about every week. So, I was thinking about buying some back links with high PR from Linkadge to lock them into place. I have noticed that my competitors have not bounced up and down but they have a PR6 vs. my PR4.

1. is Linkadage a good place to buy from? I not, where is?
2. will this hurt me in the eyes of google?
3. we sell wine and beer and I could not find a topic for that so would shopping work?
4. what should I look for when buying from one of these services.

Thank You,
Mark