Not quite. If an eBay seller is paying $20 a month for an eBay store then the max fee for selling a game is $250. Paypal, if the seller accepts it, is about 2.5%.
Where did you gather that Stern prices are higher to create more margin for distributors? Stern prices are higher because people are paying them and it makes Stern more money, period. The distributors that people on Pinside prefer to use because of their lower prices make very little on a new game and assume a lot of risk and potential headache because of the nature of the product (cost, likelihood of damage/issues, not to mention recent build quality issues...). For the record I am not a distributor (or a distributor selling buddy) but I have talked to a variety of them past and present and the margins are slim. They make the most money on lucrative trade-ins, similar to a car dealership...sorry for the comparison.