His costs are in two parts.
The actual listing costs are pretty minimal: between Free with some promotions and 30 cents unless he also chooses to use certain options.
These are paid on listing and are not refundable.
And of course there is the cost of his labour in describing, photographing and uploading the listing.
The SELLING fees (Final Value Fees) are a substantial percentage of the selling price but are refunded by eBay when you as the buyer agree to his Request for a Mutual Cancellation.
And of course leave friendly feedback is always appreciated. In addition, a pleasant transaction should get Five Stars in the Detailed Seller Ratings. These Ratings help qualify sellers for lower fees and higher placement in Search so are very important to sellers.