It's easy to see both sides of the argument and I agree with both.
I think it may help to disable the PM function of anyone with less than 10 posts as, typically, this sort of post comes from new users who are unfamiliar with the rules. Sure, you may then get folk posting e-mail addresses and phone numbers. That can't be helped but I guess they at least have to show themselves to do so. The other option is to scrap the rule but I actually think it's a good rule and would prefer to see it stay.
As Jez says, if we do the honourable thing and not engage with the seller until the rules are complied with then the chances are the collection is already gone and probably on it's way to FB or eBay.
*edit* and in all fairness, the "no paypal gifts" rule is broken by pretty much every seller, so it's not like rule breaking in this section is new.