If the buyer has already paid by PP, but is requesting pickup, REFUND the Paypal payment and ask for cash payment on pickup.
This is allowed by eBay.
The reason you don\t mix Paypal and pickup is that, in case of a Dispute, you would have no proof of delivery and would lose both the item and the payment.
BTW, in my experience it is no cheaper and no faster to use pickup than to use Canada Post. Finding a mutually convenient time and place for pickup causes significant delays and this morning I noticed gas was $1.257 a litre. Plus the buyer's time to get to the pickup spot.