I once bought tickets at the airline office in cash.

It was an Iran air office in Switzerland and they did not take credit cards because of the economic embargo.

I needed the one way outbound ticket (Shiraz to Sharjah as I recall) because I needed to show a departure for my visa and I was planning to leave by boat. One could not buy the boat ticket in advance. Iran Air had the cheapest flight - enough so that I could afford to throw it away and consider it a cost of getting the visa (and it was insurance against the ferry cancelling).