Two that come to mind are Coinbase & Circle. I did a quick google search, possibly wrong terms. The information I got was useless. I am sure this has been discussed. If there is a good article/link on this it would be appreciated.
Fee for selling and buying and enlisting new coins to their exchange.For example purchasing bitcoin with credit/debit card,coinbase charge up to 3.99% service fee.Circle has different fee structure.I have seen some exchanges even have earnings from ads