I have no dog in this fight, Carl's been ok in my book.... but if the seller was up front that the discount only applied if paid via the payment system he preferred then that's the deal. The seller didn't have to go on and on about shit.
Here's the discounted price via my preferred method of payment and here's the price if you pay via PP or CC hell... people do it here all the time.
On the flipside if the seller is a business then people wanting to pay with standard payment methods like CASH, Credit Cards or Paypal is a cost of doing business these days.