A few observations:
Yes - it can be done - and is being done - as RicardoB posted you have to think a little outside the box and stop thinking that everything is cut and dry - TGPs and LLs are not the only place to get traffic.
There are also tricks to keeping the LL traffic steady without the die factor at 6 months - although you will get a boost the first day of listing - that is usually not the buying traffic - if you read the rules at LLs you will find a way
(as put so eloquently before - hint hint)
If you really learn how to sell - which is all free sites are for - then the money will come - take a look at how people advertise in newspapers, magazine etc - you will get the answer to how to build "selling" free sites.
One disagreement - do not use sponsor hosting - most of the major Linklists will add you to their blacklists as soon as they figure out you are using it - just a fact. Pay for a virtual account.
Content - Do not use sponsor content - it lessens (and usually blocks) your chances of getting listed or will drop you down to the lowest listings - buy some content - its only $5 a set these days from the better content providers and again - Most of the good traffic Linklists are not accepting sponsor content anymore.
Last - but definitely not least - after youve built that site a day - spend another hour every day learning/reading/consuming everything you can find about search engines - they will become your best friend by the end of 6 months