on OSX lion or later, quicktime X includes mpeg2 functionality. that is why you cannot buy - you own it already.
nevertheless, streamclip needs the component.
since i always have owned quicktime pro and still have quicktime 7 on my system, i never ran into that problem.
but many people do - there is some help on the square5 website. i found the text below in a forum post, it is from square5. just check whether this is correct for your version of streamclip.
Apple states the component is unnecessary in Lion and Mountain Lion, however MPEG Streamclip still needs it.
To install the component in Lion or Mountain Lion, please download MPEG Streamclip 1.9.3b8 beta above; inside the disk image you will find the Utility MPEG2 Component M. Lion: use it to install the MPEG-2 Playback Component in Lion or Mountain Lion.
The original installer's disk image (QuickTimeMPEG2.dmg) is required.
The current versions of MPEG Streamclip cannot take advantage of the built-in MPEG-2 functionality of Lion and Mountain Lion.
For MPEG-2 files you still need to install the QuickTime MPEG-2 Playback Component, which is not preinstalled in Lion and Mountain Lion. You don't have to install QuickTime 7.