So link to one of the many android market web frontends out there, like androlib.com or my personal favorite, appbrain.com, which even has an app to allow users to install your app "from the web".
Out of curiosity, what is the chance that these front-ends will survive and thrive? It would be a pain to have to deal with a rotating series of front-ends when all you really want is a simple link directly to your app. Sometimes having and ecosystem spring up is really a sign of a broken system.
If you want a "simple link directly to your app", then use the market:// schema so that users clicking from their phone can go directly to the market page to buy it. For users on a desktop computer, show them a QR code with the market:// link so they can grab it from their phone. I'm not sure how much simpler it could get?
This does not sound simple. There should be a standard Market HTTP url which brings one to a page autopopulated with the details and the QR code of the app.
