There is a good reason to put restrictions on PMs alongside trading restrictions
If PMs are not restricted then anyone can bypass the trading restriction and go direct to a seller.
A seller decides to be nice, accepts the offer and goes through a sale, then the new dodgy member states its not received, pulls off a PayPal dispute and the seller loses out with nothing but going to court for fraud
Even before the restrictions were put into place the rules required all trading to be negotiated publicly then going to PMs for personal information on the PayPal account email, delivery address etc
This kept things out in the open should a dispute occur later on, and it gave a guide on actual negotiated selling prices to value other similar items