I am probably not as experienced as you think
but I think I can confidently anser you question.
Lets us Golf Equipmet as and example. How many golf equipment related sites sell links and have high traffic in UK? Maybe 1 or 2.
How many search engines in UK can sell you number 1 keywords for Golf Equipment and actually deliver traffic? At least 4 of them: Google, MSN, Yahoo, Ask.
Now if you have 10,000 pieces of Golf equipment that MUST be sold this Summer because after summer you will lose lots of money and if you sell all the equipment you will make 10,000 Euro profit. And to buy the keyword on Google or Yahoo cost you 5,000 Euro to sell all the equipment. To buy text link on the most popular UK golf portal will cost you 1,000 Euro, that is 4,000 less than Google or Yahoo BUT that site does not have 1/100 of the traffic Google or Yahoo can deliver. Now, which advert will you buy?
Buy AdWord for 5,000 and make 15,000 profite after paying adwords or buy 1,000 text link from the Golf equipment portal and sell only about 2,000 equipment and make 1,000 profit?
If you are a business man, which advert will you buy? Adwords or Text links?