I bought furniture today and the same thing happened the guy insisted I only pay with cash for the deal. I was haggling between a few stores and this was the best deal so I went ahead and agreed to do cash anyway (and pay when it's delivered).
Get this - he wanted to put a deposit on my card since I carry no cash on me to even pay for that. I hand him my Amex and he tried making an excuse about their cards have "5 numbers" so it won't go through and to give him a Visa or Mastercard. That was BS he just didn't want to use Amex because the processing fees are higher. I insisted he slide it because my debit card I had on me I don't know how much is in the account. He finally did an sure enough it went through. It was a mom and pop shop so I guess we should just expect unprofessional behavior from these unprofessional people.
How cheap is that - I buy $800 in furniture and and on the $100 deposit he tries haggling for Visa to save him an extra 50 cents to a dollar on processing fees.