RePost as per Chuck's recommendation...
Three really needs to be a way to publish some sort of rules/agreement. I know the announcement feature is and currently I use it. But posting the rules and forcing the user to check a box to proceed would technicaly be equivelent of a digital signature thus you can hold the user to those rules. Also wheile we are at it, there should be a way to force users to "re-agree" to the rules if necessary such as there is a change or addition to them
Probaly best way would be to have the agreement page come up when you click on post a new ad.