I have consistenly used Viagogo for my F1 tickets, and have always have a great experience doing so. I bought my Dutch GP tickets through them last year.
They always have a great selection of 1, 2 or 3-day Dutch Grand Prix tickets available, even when they’re sold out elsewhere. Plus you can often find a bargain if you keep your eyes peeled, and they have a very good ‘FanProtect guarantee’ which means your money and tickets are always safe, which is good for added peace of mind.