Free or cheap licences
Although I completely agree with the idea of being able to re-sell licences, there would still be licenses that could not be sold. These are licenses that are obtained through some other process other than buying the software. Examples of these are OEM Software (has to stay with the original equipment) and
MSDN or Technet licenced products. As these licenses are aquired at subsidized rates.
Also I would suggest that software houses would be more likely to welcome a law if it took the into account whether the version was current or not. So for example you wouldn't be able to sell the licence of a product that is the current version, but as soon as that product is replace with a new version it's licence would become sellable. This would help to protect the price of current software versions.