First read the listing again, just in case. However, if a seller has the Reserve option on the listing, it cannot be sold for less than the Reserve. Further, if the listing has a Reserve that is not met the seller does not get the fee for the Reserve back.
Tell the seller that he did not put a Reserve on the listing and that it sold for (whatever it sold for) to you.
Ask for the date and service of shipping and for the tracking number.
If he refuses to ship, go to the Resolution Centre and open an Item Not Received dispute.
OR
Go to Paypal and cancel your payment.
But leave feedback first.
The most effective feedback is calm and factual. 'Demanded more money after payment made.' At the same time you will be asked to leave Detailed Seller Ratings. A normal transaction should get five stars. The lowest Rating is ONE Star. No stars is a free pass.
You have up to 60 days to leave FB and DSRs, take your time. Negative feedback stops further cooperation.
