I'm not exactly sure if this is something we want, but it was in the mocks in bug 783669. If we do want the ability for devs to install older versions of packaged apps, this bug is to add the button and whatever else needs to happen (receipts?, etc.)
At the bare minimum we should have "Download" and "Install" I think. (And "Examine Files" which is bug 784200.)
In vidyo chat with Jonas we couldn't think of a need for this. We don't want users installing older versions. If dev's need to install older versions they should have other ways to do so and Marketplace shouldn't be blocking them. We do have a download link. I think that's sufficient.