Can you purchase a one way ticket to Denver from Boulder at the station and if so, can you use credit card?

It's also buy tickets directly on bus before start.

You can also use the RTD Mobile Tickets app and pay on the go with your credit/debit card and smart phone. Search your App Store for RTD Mobile Tickets

Yes ....buy at ticket machine....called a one way regional

Laura is correct

Yes at sales window or kiosk out side by gate one and two