eBay has the following rules for using PayPal:

"Sellers may not charge eBay buyers an additional fee for their use of ordinary forms of payment, including acceptance of checks, money orders, electronic transfers or credit cards. Such costs should be built into the price of the item."

I think they would interpret a discount for checks or money orders as a PayPal surcharge.

"Sellers who offer PayPal as a payment option in their eBay listings (either via logos or through text in the item description) must accept card funded payments. Sellers must have a Premier or Business PayPal Account or be willing to upgrade from a Personal PayPal Account to a Premier or Business Account upon receipt of a card funded payment. Sellers may not communicate to buyers that they only accept account balance or bank account funded payments, or will not accept card funded payments. eCheck is the only funding source that eBay sellers are allowed to block."