Typically, if a blog is selling links they'll fall into one of two categories: they'll be a high-powered, influential blog -- in which case, they either "nofollow" those paid links (meaning they carry no weight in Google), or they've been detected as a paid link seller by Google and their links no longer pass value, or they soon will be detected by Google and their links will stop passing value. Depending on the prominence of the blog and how the link is positioned, however, they can send some traffic to the linked page, at least in the short run.
Or they'll be a low-value crappy blog created strictly to sell advertising (links, banner ads, PPC ads, etc.) -- in which case, whether their links are "nofollowed" or discounted by Google or not won't make much difference because they don't have all that much "link juice" to pass in the first place. And those kinds of blogs typically don't send much in traffic, either. (Pretty much a waste of money, IMO.)
From a practical standpoint, the answer will be determined by your client's overall marketing strategy. (And if the client doesn't actually have a marketing strategy and instead just bounces around from one tactic to another, you can bring a whole pile of value to the table, and potentially make yourself a friend -- and referral source -- for life, by helping them recognize their need and formulate one.)
Paid links is a short-term, temporary link building tactic. If your client's strategy is to focus on the short-term, then buying links may fit into their plans. Just make sure when you buy, you're buying from reputable blogs that get a good bit of human traffic, preferably from your target audience. They'll be more expensive so you'll want to be sure track ROI in some fashion, but they at least have the potential to bring you some traffic, at least in the short run.
You do a disservice to your client, though, if you pursue a tactic if that doesn't mesh with their marketing strategy.
So if their focus is in building long-term value and ongoing traffic, I'd suggest a focus more on quality than quantity when it comes to link acquisition. In that event, zero in on getting real links from high-quality sites that are frequented by your client's target audience. These are links that are actual votes for your site, not simply links you pay for.
It is definitely more work to identify these sites, and it's MUCH harder to get links from them (that's one of the things that makes them high-quality sites -- they don't hand out links to every Tom, Dick and Harry who comes wandering down the pike or who shoots them a generic mass-email request). But links from these kinds of sites can pay dividends not only in rankings but in real, live human traffic for years to come.
Diane Aull - NineYards.com
: Helping Businesses Do Business Online
Whether you think you can, or that you can't, you are usually right.