The simple explanation, monopoly, is true. The cost of entering the financial system is prohibitively expensive. Look at the trouble Discover has had entering the market. Additionally there is collusion by the existing market players. Visa and MasterCard would not allow banks to issue a Discover Card if they issued a Visa or MasterCard. That was fought all the way to the supreme court.
Your ebay example is flawed for a couple reasons. If the fee is for fraud then charge me for buying from ebay, or other markets with high fraud. Don't charge me on everything. Name and shame ebay. Either ebay will fix the service or trustworthy escrow services will spring up at a lower price point. The other major flaw is that ebay is trivial. They have about 4B in yearly revenue. By comparison Starbucks has about 14B and Target has about 84B. Now add in Best Buy, Walmart, Hyvee, Sears, and the Gap brands. If we're talking about 93 percent of all transaction then Ebay is a nothing company, statistically zero. To set policy on it is nonsensical.
You also failed to address the selling of spending habits, which I am not compensated for.