In the case of auctioneers, this could mean increasing the buyer’s premium.
As reported by ATG in December, most auction firms currently charge between 1.5% and 3% for credit card purchases but do not charge for debit card payments.
The new rules will mean businesses can no longer add extra charges to cover credit card fees or for digital payments made via PayPal, for example.
Consumer groups estimated that the average cost of handling transactions is 0.6% but argued that companies were often charging over this amount and making a profit on surcharges.
The law has been changed after a long-running campaign by consumer rights groups to stop consumers being hit by excessive fees by firms including airlines and retailers.
Any firm caught charging customers for card payments will face fines and be ordered to repay the fees.
The EU Payment Services Directive means that such fees will be banned across the EU in 2018.