If you're selling or buying videos what is the ideal size and format these days?

Not every buyer has a computer system that can handle 1920x1080p encoded as an h.264 MP4. So do you prefer to buy and sell something more modest? For example 1280x720p encoded with Windows Media? Personally I still like the different flavours of MPEG2.

Do any of you sellers tailor the files to the individual buyers?