On eBay (and other places on the web), in the business sections you will see a number of items offering web site traffic.  Claims such as 1 million web hits to your site or other large number of traffic.  What is often not mentioned is that their site only has a few hundred or even a few thousand people visit it per month and that you will be waiting up to hundreds of years for all that traffic.  The usually promise the traffic but most do not list over what period of time.

Another trick is using an automated program to send you fake hits on your site.  This in my opinion is flat out fraud.  Watch out for any form of automation to send you traffic where actually people are not seeing the page.  Best to check your log files and watch IP addresses and who owns the IPs for starters.

Paid To Click (PTC) or Paid To Read (PTR) ads are abundant.  You need to watch out for overselling where the PTC is like above, they have sold more than they can deliver and you may be waiting for a long time to receive.  People are supposedly paid to click on them, but if they have a membership of 275 people and offering 100,000 clicks to your site, the math is fairly simple, you are only receiving at the most 275 unique clicks or they are spreading it over a large period of time.

The Paid to read (PTR) are usually email lists where your advertisement is sent out to opt-in email lists where they are paid to read the email and respond.  You want to watch out for overselling also.  Many offer to send your advertisement to (X) amount of members but do not promise you will receive that amount in traffic.  Watch you logs and you will be able to see better.  

With any of these, sometimes their click reporting is not accurate and you may have less actual click-throughs to your site than reported.  Many sites will "frame" you page and they do not know if your page actually came up in the frame, they just count that their page did.  This can cause a great deal of lost hits that you do not receive.  Monitor your logs and you can see how much you actually received.

Incentive based traffic differs from normal traffic passed through an advertisement or reference link where people come to your because they find the topic interesting.  The traffic is all paid for in some reason so they are at your site because they earn.  This is not a bad thing, but you will probably find that there is a very low ratio of visitors that actually even notice your site due to the fact they are "earning".  

A recent (to this post that is) test showed that a PTC site said there were over 3,400 visits while the logs showed a little over 2,300.  Of those that actually made it only a little less than 20 looked at more than one page and only 12 visited more than 2.  So the net gain here of real visitors would probably be 12 to 20 people.

It is not that PTC or PTR traffic is bad, you just have to balance it out with what you are actually receiving, it is not just the traffic numbers but the real visitors it generates.

The upside to this is that it can increase your rating in some search engines or site traffic evaluations.  You need to figure how much and what it is worth though.