Well you are losing out as you ARE allowed to make the charge...check with your bank

You are justified in passing the costs on to the customer...doing it by stealth is what is against the terms and conditions. If you still get repeat customers is unlikely but IT IS LEGAL...go through the ebay allowances they give the reasoning that UK sellers are allowed to do this due to english law but ONLY if they use a biz of their own (or anothers is unclear) but clearly laid out in the ebay terms and conditions.
Ever tried to actually use a Diners Club Card or American Express in the UK, either people don't take it or they charge you 5%, many have stopped doing this but the banking system still charges you higher rates for taking Amex...this is the real reason why many stores do not take it.
Minimum spend is often seen in stores regarding cheques and by default credit cards, Retail is "invitation to treat" meaning in simple terms you can charge what you like at the till, but switch selling IS illegal.
From ebay
Permitted:
A seller can add a surcharge to the final price of the item in four instances:
Credit and Debit Card Payments - Surcharging is permitted where sellers accept credit or debit cards as a means of payment for an item purchased on eBay. Such sellers will have merchant accounts with a bank or credit card company. These sellers may not pass on to the buyer a charge greater than the amount of the commission payable by the seller to the bank processing that credit or debit card payment. In other words, sellers may recoup the charges incurred as a direct consequence of accepting the credit or debit card, but no more. Please note that under English law, sellers have a legal right to pass onto the buyer the charges incurred for a credit or debit card transaction. However, this right does not apply to electronic money services. Where a credit or debit card is used to purchase electronic money to fund a transaction made via an electronic money service such as the ones listed above, this is not a credit or debit card transaction, but an electronic money service transaction.
Not permitted:
Electronic Money Services: Sellers who accept electronic money services as a means of payment for an item purchased on eBay may not impose a surcharge. Electronic money services include, but are not limited to, services such as PayPal, Nochex, FastPay or Moneybookers.
Now this is the part most people cite as the reasoning for it not being allowed....well the point is stretched to include Paypal because of the part above but only if they CAN take credit cards...if you use your credit card to put the funds into your own paypal account and THEN pay...then it becomes electronic money and NOT allowed but just click on the pay button and go through the steps and then it becomes allowable for the seller to place the charge on the bill. The way around it is to state on your auctions that you will only accept Paypal funds directly NOT funds via Paypal funded by a credit card....it shows as a credit card transfer on your Paypal statement so YOU as a seller know what the customer has done.