The only reasons I want to download something instead of buying it is because it either isn't possible to buy (because it has been deleted) or because I've heard about something but have never heard it.
The 30 second samples on iTunes, Amazon etc. are fine and easy to use but next to useless for determining if I want to buy a CD. They're great if you want to make sure you're buying the right song, assuming you've heard it before. Useless for determining if you like something.
The quality doesn't have to be brilliant. The quality of existing samples (generally low bitrate, mono, rubbish) would be enough. But good quality would better advertise the quality of the product.
I believe mySpace attempted to do something like this, but there is resistance from labels for full length versions. Various "music discovery" services have signed up for this too, but all that I've looked at have been massive horrible messes.
The main reason full previews are avoided is because it is possible to download music that is streamed via Flash, or any other method. But it takes a little bit of effort. And really, anyone willing to spend the time downloading flash audio and converting it to MP3 instead of spending a few cents to buy it instead is never going to spend any money on your product.
The other reason full length previews are rare is a rights issue. When a song is played on the radio, the composer gets broadcast fees. How is such a fee organised for a preview?
Is it really any different to listening posts at a shop?
You know, maybe I should just shut up. I seem to be on a mission to "fix" the music industry, but I'm insanely biased and am not, and probably have never been, their target market.
Perhaps I should start being a fan again. Who gives a frig what record companies do. I should be asking why bands don't have full samples of their songs on their websites, not why online shops aren't providing them...