Purchase Spain Bus Tickets on-line or during trip?

There should be no issues buying bus tickets the day of. Buses are frequent, cheap and don't often fill up. The only exception might be a major Spanish holiday.

I agree in general, but there was at least one bus I wanted to take in (northern) Spain last year that was sold out by the time I arrived at the transfer point about 30 to 60 minutes before departure. I also noticed what appeared to be demand-based pricing for some trips. I recommend not just showing up at the station 15 or 30 minutes ahead of time if if's important that you be on a particular bus. For one thing, you might run into a line at the ticket counter, and I saw signs about ticket sales ending 5(?) minutes before the scheduled departure time; they meant it, too. The larger bus companies have ticket-vending machines in major cities. They have an English-language option and are very easy to use. Except when the computer system breaks down; the lines get really ugly then.

At least some companies' machines have a print-the-schedule option in addition to ticket-seling capability. I loved that, and the first thing I did upon arrival in a new city was print the schedule for the next leg of my trip.

Bus passengers skew young. I think a lot were commuting back and forth to school (or perhaps to out-of-hometown jobs) on a weekly basis. The buses seemed busier on Sundays, Mondays and Fridays, as well as around holidays.