I walked the Levante last fall, so I saw a lot of Spain, from Valencia to Najera, on the Frances, where I volunteered as a hospitalera. I prefer to use cash and almost everywhere accepted it. The only problem which I had was in Barajas, where the buses would only take payment through pre-paid cards where the fare was deducted. As this was a change from my previous experience in Barajas and card purchase was not widely available and cash refused, I had some problems with the bus drivers. They got angry, but they didn't throw me off the bus. The situation was pretty much incomprehensible to me, as there was a double charge for transport to and from the airport, which apparently included Barajas, and it was all supposed to be paid onto cards in advance. I never did sort it out.