Buying Links

I'm considering buying one-way links to jumpstart the SE ranking some of my websites.  Price is less of an issue than the thought of somehow being penalized for it by search engines.  Theoretically if the links come from relevant sites to mine, then what does it matter whether I paid for them versus contacting the webmaster(s) directly?  Also, I paid $299 to get a Yahoo! directory listing and I can't say that it's particularly useful.  Yahoo's own Site Explorer didn't find or count it as an inbound link.  Contrast that with $249 for 10 "permanent" one-way links, I think you can see where I'm going with this.