It would probably make a good addition, aslong as people are aware of the risk when publically selling links.
It does leave you wide open for some shady character to report you to Google.
Aslong as people are discreet with the URLs they 'should' be fine.
Also to allow link trades, some people believe link exchanges are dead, its just not true, link exchanges in the form of swapping links on pages that are solely long lists of links are not particularly worthwhile, but exchanging links on 'normal' pages is still very, very effective.