Be careful on charging for it...
I know of cases where adult-websites/spammers (who have more money than they know what to do with) will buy 8 class-C's and when one gets blacklisted, they move onto the next... by the time they're on the 8th, the 1st is available again...
If you make it a policy to charge for IP, people will not pay as much attention to the "as much as you can justify" comment. If it's free, people respect the conditions.
If you can get them to respect the conditions AND pay you for it, that's the best situation...
hope this helps, -marc.
A common misconception is that people "buy" addresses. Thankfully, this is not the case, or cash would outweigh need. Groups (including the RIRs) tend to charge fees for administering allocations, but with said fee is no guarantee that the addresses won't be pulled the next day. I only wish the RIRs went through and enforced policy, pulling allocations which have been "sold"/"rented"/"leased" to others or simply being squatted on due to the original holder having disappeared or gone out of business. Maybe require allocated entities to rejustify their existing allocations every 5 years. In your situation, you are bound by the policies of the entity from which you obtain your addresses. This means that regardless of whether you charge a fee, the entity has to justify their need. This would exclude the situation you point out of a company moving back and forth between multiple /24s, since they wouldn't be able to justify the "extra" space with which they move.