it depends on what type of traffic you buy, there is an automatic traffic and there is manual traffic im speaking of traffic exchange.
traffic exchanger site is a community of business opportunity seekers that are "rewarded" to view sites,
http://www.namepros.com/npclicks.php is one type of traffic exchanger that rewards people but it's a manual traffic exchange as oppose to automatic traffic exchange which you can leave it open on your computer overnight and you gain credit and traffic. I both am an owner of automatic traffic exchanger site and a manual traffic exchanger site called ptp (paid to promote) people promote their links that randomly display urls thus if you got credits you get visitors to your site.
To people who only know ypn, adsense and parking at "sedo when it got traffic" to monetize their site this is definitely not for you.
I'm supporting manual traffic exchangers the most that delivers manual traffic/impressions/displays/visitors to your site because :imho: i got sales on my affiliate programs links. If majority of forums if not all does not allow referral/affiliate links to be posted how can someone living off affiliate programs make money after all it's by the use of this manual traffic exchangers.
social bookmarks are a different specie as it's mostly to increase backlinks to your site not deliver abrupt visitors, this is in turn long term as most of social bookmark sites like del.icio.us and stumbleupon don't delete your sites once listed on them.
manual directories are a different specie as well as it's also for backlinks and helps you get indexed on search engines.
physical shops won't pay for visitors of course and pay for ringers to ring and consume all their money on 1-800 numbers but they "incentivize" people.
what shop will you buy from the most?
a shop that when you buy you check out and there's no incentives?
or a shop that gives rebates after you buy and gives you a plus card (here in waltermart) that allows you to gain points on your purchases to be exchanged for physical items?
in this practical day and age i will tell you i will go to the shop that "incentivize" and will support it long term based on experience.
Buying traffic online cannot be compared to buying customers offline, totally different. You optimize domains for serp but you put your physical shop on high traffic locations, before you do it you're aided with a feasibility study, which answers which is the most crowded place, that my friends you already bought the place because of the "traffic" of the people found on that location.
one negative thing that I need to remove from people most probably educate them is the removal of negative outlook regarding buying traffic since most of these happened to good people who owned traffic exchangers because there are abusive people out there that abuses the traffic exchangers sites/communities whose greedy for traffic since they know it will bring them money.
successful huge sites that rely on this model of income are cj, linkshare, shareasale, clixgalore, reporting.net, primaryads.com, clickbank.com what they're selling to you are leads, people to visit or accomplish some action on your site they're not your people in the first place but take a look at some of the merchants they have. Im blessed to be paid by all affiliate programs that i've mentioned because im both an affiliate member and merchant to some of them.
i say don't jump up buying traffic but do it for free, if you want to learn just pm me.

:santa: and merry christmas in advance.