My experience with buying leads have been awful at best. So I would generally advice anyone to stay away from buying leads.
Consider...
1. If you receive an email from someone totally strange selling you some stuff, would you buy from him? What I do? I hit the spam button quick!
2. 99% of leads being sold are blacklists... Ok, I might be wrong with my stat
but seriously, think about this: After working so hard to build your list, would you risk your reputation and sell it for some quick cash? I won't and many (if not all) serious Internet marketers won't either.
3. Any guarantee that you're buying a safelist? Some lead companies sell blacklists harvested using spam robots and so YOU might be accused of spamming. Yes, this can happen even though you have, along with the email addresses, the names and other contact information of the list.
4. If any lead company tells you that the leads they sell is exclusive, they've just lied to you (except they sell a lead for $10 or more). This is no 'small' lie at all! If they can say this, then lying that their list is a safelist is no big deal for them. Nowadays, there's no such thing as an "exclusive lead" for pennies on the dollar.
Sometimes, list building can be a painfully slow process, but it's well worth it...