Browsing forums and talking with friends, I see that question popping up very often. Is the pay per click working? Should I buy traffic for my site? Do you do money with PPC? ...

I think these questions just don't make sense at all. PPC is mathematics and marketing skills. If you have a web site generating sales, and when you buy traffic, you generate more than what you spend, then you have your answer.

It's not universal science. Your product may work on some countries, not on others. The bids for your keywords might be interesting on some pay per click engines but too expensive for you on others. If you are really serious on trying PPC advertising for your site, you should decide of a budget, and spread it in different ways. $1,000 would probably be the minimum to do a serious test.
Select 5 pay per click engines, set low daily budgets, and analyze it for a week or so. Make sure you can detect on your site the sales coming from each engine and differentiate them.
Use statistics tools to monitor the behaviour of the users. If 90% of the user are just checking the first page and leaving, you know there is a problem and you can make some improvements. Try different approaches and see if you can bring more users on your "sale page".

Eliminate from your campaigns the countries that are not converting on your site.

Competition can be tough, so you might be losing money at the beginning. That's the price to pay to understand what is working for your site. Respect your initial budget and stop the campaigns that are not converting.

Some good places to start:

- Google Adwords
- Yahoo Advertising Solutions
- ExoClick.com
- MSN Advertising
- goClick.com

Good luck!