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PaymentGatewaySystem.com (very common term used for payment gateway)
GiftCardProcessor.com (gift cards grow every year because it is the easiest gift to give)
GiftCardProcessors.com
MerchantAccountToday.com (merchants want it faster they want it today)
LocalMerchantAccount.com (used to be a directory for merchant account providers)
BankcardAgent.com (used to also be a directory for agents)

MerchantItems.com (was working on but a good enough offer takes it)

Related

POSUnit.com (aka POS System)
POSUnits.com (aka POS Systems)
POSComputer.net
SmallPOS.com

CreditCardMerchantAccount.biz (very popular keyword)

Typo Names (sub-hack potential)

erchantAccount.com (merchant account)
erchantServices.com (merchant services)
reditCardProcessing.com (credit card processing)